METHOD FOR PREPARING ALKALI METAL SULPHIDE
[0001] A subject matter of the invention is a process for the preparation of
alkali metalsulfides from oxygen-comprising compounds of alkali metals and
from sulfur-comprising organic compounds.
[0002] Alkali metal sulfides are present in many and different fields of
application. For example, lithium sulfide can be used in lubricant formulations
and also as component of electrolytes in energy storage systems, such as
lithium-sulfur cells and batteries. In particular, lithium-sulfur batteries have an
improved autonomy and a greater energy density than those of lithium-ion
batteries and are thus promising candidates for new generations of energy
storage systems, for example batteries.
[0003] Furthermore, rubidium sulfide is, for example, used in applications of
semiconductor films for photovoltaic cells. Sodium sulfide is also used in
multiple applications, such as, without implying limitation, in the textile, leather
and paper industries.
[0004] Generally, alkali metal sulfides are produced by reaction between an
alkali metal compound and hydrogen sulfide. Thus, the document
JP 2010/163356 discloses a process for the production of lithium sulfide from
lithium hydroxide in an organic medium in the presence of hydrogen sulfide.
[0005] The document WO 2010/043885 discloses a process for obtaining
lithium alloy comprising transition metal sulfides by a solid-phase heat treatment
of transition metal sulfide and of lithium-comprising compound in the presence
of reducing agent, such as hydrogen sulfide.
[0006] The documents EP 0 802 159 and US 4 126 666 disclose the
production of lithium sulfide by a heat treatment of lithium hydroxide and of
lithium carbonate respectively in the presence of hydrogen sulfide or of a
mixture of hydrogen and gaseous sulfur.
[0007] All the documents of the prior art use hydrogen sulfide as sulfurization
agent. However, hydrogen sulfide is a gas, in particular the storage, handling
and post treatment of which require strict measures in terms of safety. This is
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because hydrogen sulfide is a highly toxic gas, its use on the industrial scale
represents a serious risk and it would be preferable to limit its use.
[0008] There thus exists a major need to be able to have available a process
for the industrial production of alkali metal sulfides which does not exhibit the
disadvantages of the prior art.
[0009] Surprisingly, the applicant company has discovered, after various
experiments and handling operations, that the use of certain sulfur-comprising
organic compounds makes it possible to carry out the reaction for the
production of alkali metal sulfides, in or not in the presence of catalyst, under
conditions which can be easily achieved industrially, while not resorting to toxic,
indeed even highly toxic, gaseous reactants, such as hydrogen sulfide.
[0010] According to a first aspect, the present invention thus relates to a
process for the preparation of an alkali metal sulfide comprising at least one
stage a) of reaction of at least one oxygen-comprising compound of said alkali
metal with at least one sulfur-comprising compound of formula (I):
R-S-Sx-R'
(°)n
(i)
in which:
- R represents a linear or branched alkyl or linear or branched alkenyl radical
comprising from 1 to 6 carbon atoms, preferably from 1 to 4 carbon atoms,
limits included;
- n is equal to 0, 1 or 2;
- x is equal to 0 or to an integer taking the values from 1 to 10, limits included;
preferably, x is an integer equal to 1, 2, 3 or 4;
- R' represents a linear or branched alkyl or linear or branched aikenyl radical
comprising from 1 to 6 carbon atoms, limits included, preferably from 1 to 4
carbon atoms, or, only if n = x = 0, a hydrogen atom;
- or else R and R' can together form, and with the sulfur atom(s) which
carry(ies) them, a sulfur-comprising heterocycle comprising from 2 to 12 carbon
atoms, preferably from 2 to 8 carbon atoms, limits included, and optionally, in
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addition to the sulfur atom(s) indicated in the formula (I), one or more
heteroatoms chosen from oxygen, nitrogen and sulfur.
[0011] Mention may be made, among the abovementioned sulfur-comprising
heterocycles, as nonlimiting examples, of thiophene, thiolane, dithiolane,
thiazole, thiazine, thiepane, dithiepane, oxathiane and others, to mention only
the commonest of them, but also the suifur-comprising cyclic derivatives of
terpenes, such as, for example, the addition products of sulfur to terpenes, in
particular to myrcene, which are mono-, di-, tri- or tetrathioperillenes.
[0012] The compounds of formula (I) which can be used in the context of the
process of the present invention exhibit numerous advantages, among which
may be mentioned, without implied limitation, of not being gaseous at ambient
temperature and generally being liquid at ambient temperature, and also of
being much less toxic than hydrogen sulfide, indeed even of being only slightly
toxic or nontoxic. Contrary to the handling of toxic and gaseous hydrogen
sulfide, the use of the compounds of formula (I) in the liquid or solid form thus
makes it possible to facilitate in particular the handling procedures and also the
general way in which the process for the synthesis of the alkali metal sulfides is
carried out.
[0013] Among the compounds of formula (I), preference is given to the
compounds of formula (I) for which n and x do not simultaneously represent the
value 0. According to another preferred embodiment, preference is given to
those for which n represents 1 and, among these, preference is given to those
for which x represents 0 and, in this case, the compounds of formula (I) are
sulfoxides, a particularly preferred representative of which is dimethyl sulfoxide
(DMSO).
[0014] According to another embodiment, preference is given to the
compounds of formula (I) for which n represents 2 and, among these,
preference is given to those for which x represents 0 and, in this case, the
compounds of formula (I) are sulfones, a representative of which particularly
suited to the process of the present invention is tetramethylene suifone or
dimethyl suifone.
[0015] According to a very particularly preferred embodiment, the compound(s)
of formula (I) employed in the process of the present invention are the
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compounds of formula (I) for which n represents 0, that is to say sulfides,
disulfides or polysulfides, according to whether x represents 0 or 1 or an integer
strictly greater than 1 respectively.
[0016] According to an advantageous embodiment of the present invention,
the sulfur-comprising compound of formula (I) is such that n is equal to 0 and x
= 1, 2, 3 or 4, preferably x = 1, 2 or 3, preferably x = 1 or 2 and entirely
preferably x = 1. According to yet another embodiment, the compound of
formula (I) can be a mixture of compounds of formula (I) for which, on average,
x is between 2 and 10 (limits included), preferably with a mean value of x of
between 3 and 5 (limits included).
[0017] In one embodiment, the sulfur-comprising compound of formula (I) is
such that R and R' each represent a linear or branched alkyl radical comprising
from 1 to 6 carbon atoms, preferably from 1 to 4 carbon atoms, for example
methyl, ethyl, propyl (n-propyl or isopropyl), butyl (n-butyl, isobuty! or tert-butyl),
pentyls and hexyls. In addition, preference is given to the compounds of formula
(I) in which the R and R' radicals are identical.
[0018] Preferably, the sulfur-comprising compound is chosen from dimethyl
sulfide, diethyl sulfide, di(n-propyl) sulfides, diisopropyl sulfide, di(n-butyl)
sulfide, diisobutyl sulfide or di(tert-butyl) sulfide. According to yet another
preference, the sulfur-comprising compound is chosen from dimethyl disulfide,
diethyl disulfide, di(n-propyl) disulfide, diisopropyl disulfide, di(n-butyl) disulfide,
diisobutyl disulfide, di(tert-butyl) disulfide, dimethyl trisuifide, diethyl trisuifide,
di(n-propyl) trisuifide, diisopropyl trisuifide, di(n-butyl) trisuifide, diisobutyl
trisuifide, di(tert-butyl) trisuifide, dimethyl tetrasulfide, diethyl tetrasulfide, di(npropy!)
tetrasulfide, diisopropyl tetrasulfide, di(n-butyl) tetrasulfide, diisobutyl
tetrasulfide, di(tert-butyl) tetrasulfide and their mixtures, preferably dimethyl
disulfide, diethyl trisuifide and dimethyl tetrasulfide, and also mixtures of
symmetrical or asymmetrical (R and R' respectively identical or different) dialkyl
polysulfides (n = 0 and 1 < x < 10), such as, for example, the mixtures known
under the acronym DSO (Disulfide Oils).
[0019] The compounds indicated above can, of course, be used alone or as
mixtures, for example as mixtures of two or more of the sulfur-comprising
compounds listed above in all proportions.
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[0020] In a preferred embodiment of the invention, the sulfur-comprising
compound is chosen from dimethyl disulfide (DMDS), diethyl disulfide (DEDS)
and their mixtures.
[0021] The sulfur-comprising compounds of formula (I) are known and are
commercially available or are easily prepared from procedures known from the
patent literature, from the scientific literature, from Chemical Abstracts or from
the Internet.
[0022] In the process according to the present invention, the sulfur-comprising
compound(s) as just defined above is (are) brought into contact with at least
one oxygen-comprising alkali metal compound. In the present description,
"oxygen-comprising alkali metal compound" is understood to mean a compound
comprising at least one aikali metal atom and at least one oxygen atom.
[0023] Preferably, the oxygen-comprising alkali metal compound is chosen
from the oxides, hydroxides, hydrogencarbonates, carbonates, sulfates,
sulfides, nitrates, nitrites and carboxylates (for example oxalates, formates,
acetates, lactates, citrates and others) of said alkali meta! and the mixtures of
two or more of them, in all proportions.
[0024] "Aikali metal" is understood to mean the alkali metals of Group 1 of the
Periodic Table of the Elements and more particularly the alkali metals chosen
from lithium, sodium, potassium, rubidium, cesium and their mixtures;
preferably, the alkali metal is lithium, sodium or potassium; entirely preferably,
the alkali metal is lithium.
[0025] As regards the oxygen-comprising lithium compounds, preference is
given to those chosen from lithium oxide (Li20), lithium hydroxide (LiOH),
lithium carbonate (Li2C03), lithium hydrogencarbonate (LiHC03), lithium sulfate
(Li2S04), lithium nitrate (LiN03) and lithium oxalate (IJ2C2O4).
[0026] As regards the oxygen-comprising sodium compounds, preference is
given to those chosen from sodium oxide (Na20), sodium hydroxide (NaOH),
sodium carbonate (Na2C03), sodium hydrogencarbonate (NaHC03), sodium
sulfate (Na2S04), sodium nitrate (NaN03) and sodium oxalate (Na2C204).
[0027] As regards the oxygen-comprising potassium compounds, preference is
given to those chosen from potassium oxide (K20), potassium hydroxide (KOH),
potassium carbonate (K2C03), potassium hydrogencarbonate (KHC03),
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potassium sulfate (K2SO4), potassium nitrate (KN03) and potassium oxalate
(K2C204).
[0028] As regards the oxygen-comprising rubidium compounds, preference is
given to those chosen from rubidium oxides (Rb20, Rb202, Rb203l Rb02),
rubidium hydroxide (RbOH), rubidium carbonate (Rb2C03), rubidium
hydrogencarbonate (RbHC03), rubidium sulfate (Rb2S04), rubidium nitrate
(RbN03) and rubidium oxalate (Rb2C204).
[0029] As regards the oxygen-comprising cesium compounds, preference is
given to those chosen from cesium oxide (Cs20), cesium hydroxide (CsOH),
cesium carbonate (Cs2C03), cesium hydrogencarbonate (CsHC03), cesium
sulfate (Cs2S04), cesium nitrate (CsN03) and cesium oxalate (Cs2C204).
[0030] The oxygen-comprising compounds of alkali metals as just described
above are known and are available commercially or are easily prepared from
procedures known from the patent literature, from the scientific literature, from
Chemical Abstracts or from the Internet.
[0031] The process according to the present invention is very particularly
suitable for the preparation of lithium sulfide (Li2S) from dimethyl disulfide
(DMDS) and lithium hydroxide or lithium carbonate or lithium oxide, preferably
from DMDS and lithium oxide and/or lithium hydroxide.
[0032] According to the process of the present invention, the "sulfur-comprising
compound(s)/oxygen-comprising alkali metal compound(s)" molar ratio is
chosen such that the sulfur/alkali metal molar ratio is generally between 0.5 and
10, preferably between 0.5 and 5, limits included. The implementation of the
process of the invention with said molar ratio of less than 0.5 would not result in
a complete sulfurization of the alkali metal(s). A molar ratio of greater than 10
would not contravene the implementation of the process of the invention but
would be regarded as nonprofitable since a large portion of the compound(s) of
formula (I) would not be used for the sulfurization of the alkali metal.
[0033] The process according to the present invention is characterized in that it
comprises at least one stage a) of reaction of at least one compound of formula
(I) as defined above with at least one oxygen-comprising alkali metal compound
as defined above.
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[0034] According to an alternative form of the process of the invention, stage
a) aiso comprises the addition of an amount of hydrogen in a H2/sulfur atom
originating from the sulfur-comprising compound of formula (I) molar ratio of
between 0.01 and 10, preferably between 0.01 and 1.
[0035] Preferably, stage a) of the process of the invention is carried out at a
temperature of between 150°C and 1500°C, more preferably between 150°C
and 800°C.
[0036] According to one embodiment of the process of the invention, stage a)
is carried out at a temperature of between 150°C and 400°C, preferably of
between 200°C and 350°C.
[0037] In this first temperature range, stage a) is advantageously carried out in
the presence of at least one catalyst which has in particular the aim of
increasing the kinetics of the reaction.
[0038] In this case, the catalyst can be of any type known to a person skilled in
the art and preferably chosen from cobalt oxides, nickel oxides, molybdenum
oxides and their mixtures, which are or are not supported, for example on silica,
alumina or active charcoal. For example, the catalyst can be chosen from the
commercial catalysts from Axens, such as HR626, HR526, HR548 or HR648.
[0039] According to another embodiment of the invention, stage a) is carried
out a temperature preferably of between 300°C and 800°C, more preferably still
between 300°C and 600°C. In this second temperature range, the reaction can
be carried out without catalyst and is preferably carried out in the absence of
catalyst. However, stage a) can be carried out in the presence of at feast one
catalyst, such as those defined above, in this temperature range of 300 and
600°C.
[0040] The reaction between at least one compound of formula (I) and at least
one oxygen-comprising alkali metal compound can be carried out in a solvent
medium or in the absence of solvent; however, it is preferable to carry out the
reaction in the absence of solvent in order to avoid possible treatments and/or
recyclings of the solvent(s) used.
[0041] Stage a) of the process according to the invention can be carried out
under pressure, under reduced pressure or also at atmospheric pressure,
depending on the temperature chosen, the nature of the reactants, the nature of
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the final product desired and the presence of the optional solvent and of the
optional catalyst. Generally, stage a) is preferably carried out at atmospheric
pressure, for obvious reasons of simplicity and of the overall cost of
implementing the process of the invention.
[0042] Stage a) of the process of the present invention can be carried out in
any type of suitable reactor, with stirring or without stirring, or else in a reaction
column, the latter embodiment being particularly preferred. At least one sulfurcomprising
compound of formula (I), preheated or not preheated, is injected, in
the liquid form, continuously or batchwise, directly or via a vaporizer if the
compound of formula (I) is in the gas form, onto at least one oxygen-comprising
alkali metal compound, generally in the solid form, preheated or not, in the
optional presence of one or more solvent(s), optionally in the presence of
hydrogen and in the optional presence of one or more catalysts, as indicated
above.
[0043] The reaction is carried out at the chosen temperature and at the chosen
pressure for a period of time sufficient to obtain the desired degree of
sulfurization of said oxygen-comprising alkali metal compound, generally for a
period of time of between a few seconds and a few hours.
[0044] In one embodiment of the invention, stage a) also comprises the
addition of an amount of water such that the H20/sulfur atom originating from
the sulfur-comprising compound of formula (I) molar ratio (that is to say, moles
of H20/moles of S ratio) is advantageously between 0.01 and 10, preferably
between 0.01 and 1. In an alternative form, the water can be completely or
partially replaced with hydrogen, in the same proportions as those indicated
above.
[0045] The water and/or the hydrogen can be added all at once or on several
occasions, continuously or noncontinuously. In a preferred embodiment, water
is added during stage a) of the process according to the present invention.
[0046] According to an alternative form of the process of the invention, sulfur
can be contributed to or introduced into the reaction medium. The sulfur can
originate from the sulfur-comprising compound itself or also be added (injected)
directly in the liquid or solid form to (into) the reaction medium.
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[0047] When sulfur is directly injected in the liquid form into the reaction
medium, it is thus possible to envisage carrying out the process of the invention
by using DMDS and liquid sulfur (S8). In an alternative form, the sulfur can be
introduced in the solid form, for example as an intimate mixture of solid sulfur
with the oxygen-comprising alkali metal compound, the sulfide of which it is
desired to prepare.
[0048] It should also be understood that the process of sulfurization of at least
one oxygen-comprising alkali metal compound according to the present
invention can be carried out in the presence of carbon disulfide (CS2) and/or of
hydrogen sulfide (H2S), although this does not constitute a preferred
embodiment, in particular for the reasons mentioned above of toxicity and of
dangerousness of these compounds.
[0049] In one embodiment of the invention, stage a) is followed by a stage b)
of recovery of the alkali metal sulfide prepared in the reactor or the column used
in stage a) and optionally by a stage c) of purification of the alkali metal sulfide
obtained, according to any method known to a person skilled in the art, for
example by washing, recrystallization and others.
[0050] According to another aspect, the present invention relates to the use of
at least one compound of formula (I) as defined above in the preparation of
alkali metal sulfides of formula M2S, in which M represents an atom of an alkali
metal preferably chosen from lithium, sodium, potassium, rubidium and cesium,
and in particular to the use of dimethyl disulfide in the preparation of lithium
sulfide.
[0051] As indicated above, alkali metal sulfides, such as those obtained
according to the process of the present invention, have applications in a great
many fields, such as, for example, semiconductor films for photovoltaic cells in
the case of rubidium sulfide, in the textile, leather and paper industries for
sodium sulfide, or also as component of lubricant formulations or component of
electrolytes or of electrodes in energy storage systems, in the case of lithium
sulfide.
[0052] Lithium sulfide is in particular entirely suitable as component in energy
storage systems, such as lithium-sulfur cells and batteries, which exhibit an
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improved autonomy and a greater energy density than those of lithium-ion
batteries.
[0053] This is because lithium-ion batteries can exhibit certain safety problems
due to the use of a negative electrode made of lithium metal. The use of such a
negative electrode and the safety problems which are inherent in it can be
solved by virtue of the use of a positive electrode based on lithium sulfide.
[0054] This is because this alternative makes it possible to produce Li-ion/S
storage batteries, in that the use of an Li2S positive electrode contributes the
lithium source and can then be combined with a negative electrode material
other than lithium, such as, for example, graphite (C), silicon (Si), tin (Sn), and
the like.
[0055] The following examples illustrate the invention without limiting in any
way the scope of protection applied for as it appears in the appended claims.
Example 1:
[0056] Powdered lithium oxide (Li20) with a particle size of 150 pm and with a
purity of 99.5% from Alfa Aesar is milled so as to recover a particle size of
approximately 10 urn. 10 g of this lithium oxide powder are withdrawn and are
deposited in a silicate crucible.
[0057] A tube made of Hastelloy with a diameter of 3 cm and a length of 50 cm
was especially designed in order to place and maintain a perforated screen at
30 cm from one edge of the tube (and thus 20 cm from the other edge). The
30 cm portion of the tube is referred to as part A and the 20 cm portion of the
tube as part B in order to understand this example. The two ends of the tube
are intended to accept perforated screens before connecting this tube to a gas
supply network.
[0058] The silicate crucible containing the 10 g of lithium oxide is set down at
the center of part B of the tube. The tube is inserted into a pyrolysis oven in
order to be able to drop in temperature.
[0059] The ends of the tube are connected to gas supply networks which make
it possible to supply the tube with gas in the direction from part A to part B, after
having taken the precaution of positioning the perforated screens at the 2 ends
of the tube. A stage of dehydration of the lithium oxide is undertaken by carrying
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out flushing under nitrogen at 250°C for 10 minutes. The nitrogen flow rate is
50 Sl/h. At the end of this stage, the temperature is increased to 550°C] still
under a nitrogen flow.
[0060] Subsequently, dimethyl disulfide (DMDS), supplied by Arkema, with a
purity of 99.5%, is injected into the tube. A flow rate of 60 g/h of DMDS in
60 Sl/h of nitrogen is injected into the tube in order to sulfurize the lithium oxide
powder to give lithium sulfide. The reaction time is maintained for 2 hours. At
the end of the suifurization, the reactor is cooled under nitrogen with a flow rate
of 50 Sl/h.
[0061] X-ray fluorescence analysis of the final powder obtained reveals an
acceptable degree of suifurization corresponding to a molar degree of
conversion of Li20 to give Li2S of greater than 95% of Li2S formed. However, a
few traces of carbon-comprising compounds are observed by elemental
analysis.
Example 2:
[0062] Example 1 is repeated but, for this example, the suifurization reaction is
carried out with a flow rate of 60 g/h of DMDS in 60 Sl/h of hydrogen (instead of
nitrogen). The DMDS/H2 mixture is injected into the tube in order to sulfurize the
lithium oxide powder to give lithium sulfide. The reaction time is maintained for 2
hours. At the end of the suifurization, the reactor is cooled under nitrogen with a
flow rate of 50 Sl/h.
[0063] X-ray fluorescence analysis of the final powder obtained reveals a
complete molar degree of conversion of Li20 to give Li2S of 100%.
Example 3:
[0064] Example 2 is repeated but, for this example, a cobalt/molybdenum
catalyst supported on alumina, HR626 from Axens, is positioned throughout the
whole of compartment A of the Hastelloy tube. A total amount of approximately
210 mi of catalyst is thus positioned between the two perforated screens
separating parts A and B and the other at the end of the tube.
[0065] Instead of lithium oxide, as in example 2, lithium hydroxide (LiOH)
powder with a purity of 99.995% is supplied by Alfa Aesar. This LiOH is milled in
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order to recover a particle size of approximately 10 urn. 10 g of this lithium
hydroxide powder are withdrawn and are deposited in a silicate crucible. The
silicate crucible containing 10 g of lithium hydroxide is set down at the center of
part B of the tube.
[0066] The tube is inserted into a pyrolysis oven in order to be able to go up in
temperature. The ends of the tube are connected to gas supply networks which
make it possible to supply the tube with gas in the direction from part A to part
B, after having taken the precaution of positioning the perforated screens at the
2 ends of the tube. A stage of dehydration of the lithium hydroxide is undertaken
by carrying out a flushing with nitrogen at 250°C for 1 hour. The nitrogen flow
rate is 50 Sl/h.
[0067] At the end of this stage, the temperature is increased to 350°C, still
under a nitrogen flow. Subsequently, a flow rate of 40 g/h of DMDS in 40 Sl/h of
hydrogen is injected into the tube in order to sulfurize the lithium hydroxide
powder to give lithium sulfide. The reaction time is 30 minutes. At the end of the
sulfurization, the reactor is cooled under nitrogen with a flow rate of 50 Sl/h.
[0068] X-ray fluorescence analysis of the final powder obtained reveals a
complete molar degree of conversion of 2LiOH to give Li2S of 100%, this being
the case despite a sulfurization temperature of 350°C.

CLAIMS
1. A process for the preparation of an alkali metal sulfide from at least one
oxygen-comprising alkali metal compound, comprising at least one stage a)
consisting in reacting said oxygen-comprising alkali metal compound(s) with at
least one sulfur-comprisingcompound of formula (I):
R—S—Sx-R'
(i)
in which:
- R represents a linear or branched aikyl or linear or branched alkenyl radical
comprising from 1 to 6 carbon atoms, preferably from 1 to 4 carbon atoms,
limits included;
- n is equal to 0, 1 or 2;
- x is equal to 0 or to an integer taking the values from 1 to 10, limits included;
preferably, x is an integer equal to 1, 2, 3 or 4;
- R' represents a linear or branched alkyl or linear or branched alkenyi radical
comprising from 1 to 6 carbon atoms, limits included, preferably from 1 to 4
carbon atoms, or, only if n = x = 0, a hydrogen atom;
- or else R and R' can together form, and with the sulfur atom(s) which
carry(ies) them, a sulfur-comprising heterocycle comprising from 2 to 12 carbon
atoms, limits included, preferably from 2 to 8 carbon atoms, limits included, and
optionally, in addition to the sulfur atom(s) indicated in the formula (I), one or
more heteroatoms chosen from oxygen, nitrogen and sulfur.
2. The process as claimed in claim 1, in which said oxygen-comprising
alkali metal compound is chosen from the oxides, hydroxides,
hydrogencarbonates, carbonates, sulfates, sulfites, nitrates, nitrites and
carboxylates of said alkali metal and also the mixtures of two or more of them,
in all proportions.
3. The process as claimed in claim 1 or claim 2, in which the alkali metal is
chosen from lithium, sodium, potassium, rubidium, cesium and their mixtures;
preferably, the alkali metal is lithium.
4. The process as claimed in any one of the preceding claims, in which
said sulfur-comprising compound of formula (I) is such that n = 0.
5. The process as claimed in any one of the preceding claims, in which
said sulfur-comprising compound of formula (I) is such that x = 1, 2 or 3 or is a
mixture of sulfur-comprising compounds with, on average, x between 2 and 10
(limits included), preferably with a mean value of x of between 3 and 5 (limits
included).
6. The process as claimed in any one of the preceding claims, in which the
sulfur-comprising compound is chosen from dimethyl trisulfide, diethyl trisulfide,
dimethyl tetrasulfide, diethyl tetrasulfide, dimethyl disulfide, diethyl disulfide,
di(n-propyl) disulfide, diisopropyl disulfide and their mixtures, preferably
dimethyl disulfide, diethyl trisulfide and dimethyl tetrasulfide, and their mixtures.
7. The process as claimed in any one of the preceding claims, in which
stage a) is carried out at a temperature of between 150°C and 1500°C,
preferably of between 300°C and 800°C and more preferably of between 300°C
and 600°C.
8. The process as claimed in any one of the preceding claims, in which
stage a) is carried out in the presence of at least one catalyst chosen from
cobalt oxides, nickel oxides, molybdenum oxides and their mixtures, which are
or are not supported, for example on silica, alumina or active charcoal, at a
temperature of between 150 and 400°C, preferably of between 200 and 350°C.
9. The process as claimed in any one of claims 1 to 7, in which stage a) is
carried out in the absence of catalyst at a temperature preferably of between
300 and 600°C.
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10. The process as claimed in any one of the preceding claims for the
preparation of lithium sulfide (Li2S) from dimethyl disulfide (DMDS) and lithium
hydroxide or lithium carbonate or lithium oxide, preferably from DMDS and
lithium oxide and/or lithium hydroxide.
11. The use of at least one compound of formula (I) as claimed in any one
of claims 1,4, 5 and 6 in the preparation of an alkali metal sulfide o{ formu(a
M2S, where M represents an alkali metal, preferably chosen from lithium,
potassium, sodium, rubidium and cesium, preferably from lithium, potassium
and sodium.
12. The use as claimed in claim 11, in which the compound of formula (I) is
dimethyl disulfide and the alkali metal sulfide is lithium sulfide.