Note: Descriptions are shown in the official language in which they were submitted.
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BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a process for the selective mono-
halogenation of alkanes to alkyl monohalides in a homogeneous (sup-
ported or unsupported) liquid phase system under mild conditions of
temperature and pressure. The halogenation may be followed by hydrol-
ysis to produce alcohols.
Description of Related Art
Selective halogenation of alkanes to alkyl halides, parti-
cularly chlorination to alkyl chlorides under relatively mild tempera-
ture and pressure offers the possibility for development of simple,
low cost means for producing alkyl halides. Alkyl halides are known
to those having ordinary skill in the art to have utility as a feed-
stock for more valuable commercial reactions. For example, methyl
chloride and other alkyl halides have utility as an intermediate for
product;on of alcohols, such as methanol, which itself is useful as an
alternative, less environmentally damaging, fuel source. Additional-
ly, methanol can be used as a feedstock for chemical reactions; for
example, it can be used in reactions to yield gasoline or other hydro-
carbons. More importantly, alcohol can be used as a transportation
fuel source or as an additive to transportation fuels, particularly
gasoline, to reduce hydrocarbon emissions and produce a more environ-
mentally safe fuel.
The literature describes a number of processes for monohalo-
genating alkanes. However, unlike the processes described in the
literature, applicants' monohalogenate alkanes in a process that uses
a homogeneous liquid phase/transition metal complex system that uses
no metal-bound halogen sources operates in the presence of the transi-
tion metal halide complexes, shows no evidence of metallic platinum
formation during the reaction until the added halogen source is
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depleted, and is highly selective for monohalogenated products and
their hydrolysis products.
SUMMARY OF THE INVENTION
The present invention relates to a process for selectively
producing alkyl monohalides, particularly alkyl chlorides, at rela-
tively mild conditions of temperature and pressure in a homogeneous
system from alkanes, particularly methane and ethane and mixtures
thereof, by combining the alkane and an added halogen source in an
aqueous solution, preferably water, in the presence of a soluble
transition metal halide complex. The alkyl monohalides may be con-
verted via hydrolysis to alcohols.
DESCRIPTION OF THE INVENTION
Alkyl halides, particularly alkyl chlorides such as methyl
chloride, can be produced selectively by a process which comprises
adding an alkane or mixtures of alkanes, and an added halogen source
to an aqueous solution in the presence of a transition metal halide
complex. The alkanes suitably may be methane, ethane or higher
alkanes. The halogens are preferably chlorine, fluorine or bromine.
The corresponding alcohol(s) may be produced during the reaction from
the hydrolysis of the resulting alkyl halides or in a separate hydrol-
ysis step.
The transition metal halide complex can be added to the
system or may be produced by one ordinarily skilled in the art, in
situ, from a compound consisting of a transition metal and a ligand
capable of reacting to form the complex provided that the resulting
complex is homogeneous with the system. Particularly useful transi-
tion metal halide complexes are those in which the transition metal is
platinum, palladium, and nickel or mixtures thereof or, more prefera-
bly, platinum; and the halide is fluoride, chloride, bromide, or
iodide or mixtures thereof, preferably chloride. The preferable
combination of transition metal and halide is that of platinum and
chloride; for example, as (PtC14)-2/(PtC16)-2 or as (PtC14)-2 alone.
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The cation portion of the complex may be a Group IA or IIA element,
preferably H+, K+, or Na+. The particular complex may be prepared by
methods known to ones ordinarily skilled in the art or obtained from
commercial sources. The complex should be homogeneous with the
aqueous solution in the system.
The complex useful in the process of the present invention
and the aqueous solution also may be supported in a solid hydrophilic
support. In general, the support will be a porous solid material.
For example, materials having a pore volume relative to solid weight
of from about 0.1 to 1.5 cubic centimeters per gram, with a preferred
range of from about 0.4 to 1.0 are especially useful supports. The
macropore volume of the porous support should be at least 10% of the
total pore volume. By macropore volume is meant pores having diame-
ters greater than 100 Angstroms.
Specific examples of such materials useful as a support for
the homogeneous liquid phase system in the practice of the process of
the present invention are silica, clay, alumina, silica/alumina, acid
treated clay and titania. Indeed, it is particularly preferred in the
practice of the present invention to use acidic porous support materi-
als such as silica/alumina, clay and, even more particularly, acid
treated clay.
In a supported homogeneous liquid phase system, the transi-
tion metal complex used in the process of the present invention is
dissolved in a supported aqueous ac;d phase; i.e., an aqueous acid
phase that does not circulate or flow as a liquid, but is immobile and
supported by the porous solid support. Typical supported aqueous acid
liquid phase materials that may be used in the practice of the present
invention include aqueous solutions of HCl, HF, CH3COOH, CF3COOH,
H3P04, H2S04, CH3S03H, CF3S03H, BF3 and mixtures thereof, preferably
HCl, HF and mixtures thereof.
The volume of the supported aqueous phase will generally be
a predetermined maximum amount that can be supported without causing
the particles of the support to stick together, which amount may
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readily be determined by one ordinarily skilled in the art. The
amount of aqueous phase should be less than that of the pore volume of
the specific support employed. Indeed, it is preferred that the
amount of aqueous phase will be about 10% less than the pore volume of
the support. Thus, for example, about 1.3 cc of aqueous phase will be
used with a support having a pore volume of 1.5 cc/gm.
Finally, in the homogeneous liquid phase system used in the
process of the present invention, there is included a transition metal
halide complex which is dissolved in the aqueous phase. The transition
metals that may be employed herein include cobalt, rhodium, iridium,
palladium, platinum, ruthenium, rhenium and mixtures thereof, but
prefe~rred is platinum.
In preparing a supported homogeneous liquid phase system,
the transition metal complex is first dissolved in the aqueous phase,
then the solution is impregnated into the porous support material by
any appropriate means known to one skilled in the art, for example, by
the incipient wetness technique.
The added halogen source used in the practice of the present
invention should be additional to any halide contained in the transi-
tion metal halide complex having the transition metal halide. The
source may be any Group VIIA element or compound, any Group VIIA-
containing compound, and any other Group VIIA-containing species that
exist(s) as an equilibrium product of the reaction of the Group VIIA
element, compound or Group VIIA-containing species in water in the
presence of the complex and mixtures thereof. The added halogen
source may be in elemental, molecular, ionic, free radical, other form
or species, or mixtures thereof that are consistent with the chemical
composition of the source. It may be introduced into the system in
gaseous, liquid or other form that is or becomes soluble or dissolves
in whole or in part in water, or it may be present in the system as an
equilibrium product of the reactions involved in the particular
system. The added halogen source is preferably a halogen, halide and
hypohalide, or mixtures thereof, more preferably chlorine, a chloride
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and a hypochloride; even more preferably, Cl2 and HOCl, most prefera-
bly Cl2-
In the process of the present invention, monohalogenation ofthe alkanes is carried out under relatively mild conditions. ~uitable
monohalogenations may be carried out at a temperature range from about
20C to about 315C, preferably from about 20C to about 200C, more
preferably from about 25 to about 150C; and hydrolysis to the
corresponding alcohols may be accomplished at temperatures from about
20C to about 315C, preferably from about 100C to about 250C, more
preferably from about 100C to about 150C. The total pressure
selected will vary based on the form in which the alkane and added
halogen source are introduced into the system (e.g. liquid, gas), but
generally for gaseous sources should be from about 1 atm to about 300
atm. Where, for example, the reaction is carried out using gaseous
Cl2 at about 20C to about 25C, the preferable pressure range is from
about 1 to about 6 atm. Halogenation of alkanes to alkyl monohalides,
according to the process of the present invention, may be carried out
selectivelyS using the ratio of added halogen source to alkane of
greater than or equal to about 1:1, preferably from about 1:1 to about
1:10, more preferably from about 1:1 to about 1:100.
If a significant amount of alcohol production is desired the
reaction should be permitted to progress for a sufficient time to
produce alcohol, but should not be allowed to proceed to the point
that the added halogen source is depleted. The reactants should be
used in effective amounts for the production of the alkyl halide.
Where the added halogen source is Cl2 or ~IOCl and the complex contain-
ing the transition metal halide is Na2PtCl4/Na2PtCl6 or Na2PtCl4
alone, the process produces alkyl chlorides, particularly methyl
chloride and ethyl chloride in high selectivity at pressures as low as
about 5 atm, and at temperatures as low as about 100C almost immedi-
ately. The reaction may be expected to proceed slowly, even at
pressures as low as 1 atm.
In all cases, the pressure and temperature of the reaction
and concentrations of reactants should be such that the flash point of
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the gaseous reactants is not exceeded. Due regard should be given to
the corrosive nature of the particular Group VIIA reactants used.
Particularly in the case of fluorinati~n of alkanes, reactions should
be performed in high dilution, preferably in the presence of an inert
gas, to minimize handling problems. For processes known to those
ordinarily skilled in the art for carrying out reactions using halo-
gens, see e.g., F. Cotton and G. Wilkinson, Advanced Inorqanic
ChemistrY. A Comprehensive Text, 4th ed., Part 2, Ch. 17, p.p.
542-576 "The Group VII Elements, Fluorine, Chlorine, Bromine, Iodine
and Astatine".
The process of the present invention may be run in batch or
may be operated continuously. The latter may be accomplished by
removing on an ongoing basis the alkyl monohalides (or the alcohol, if
the reaction is allowed to proceed under conditions which result in
the hydrolysis to alcohol), recycling the complex containing the
transition metal halide and regenerating the halogen source by oxida-
tion of the halogen cnntaining by-products of the reaction.
Reaction times for the process of the present invention will
depend on the particular combination of reagents used, the sample
size, and the type of process (batch or continuous), but should be
sufficient to permit the synthesis of alkyl monohalides, and, if
alcohol production is desired, the hydrolysis of the alkyl monohalides
to alcohol. In order to maintain the selectivity of the reaction
toward the production of alkyl monohalides, care should be taken such
that the added halogen source is not depleted.
Subject to the foregoing limitations, reaction times gener-
ally needed to produce alcohol in quantities that are equal to or
greater than those of, for example, alkyl chloride, are from about 15
minutes to about 16 hours at from about 100C to about 150C and from
about 20 atm to about 80 atm, the more preferable time being from
about 30 minutes to about 8 hours.
The selection of the particular reaction times, conditions
and combination and concentrations of reagents will be readily
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apparent to one ordinarily skilled in the art given parameters estab-
lished by the the teachings herein. General background concerning,
for example, the conditions necessary for the chlorination of methane
can be found in J. S. Sconce, Chlorine. Its Manufacture. Properties
and Uses, R. Landau and S. Fox, Chapter 12, "Chlorinated Methanes",
pp. 334 to 375. Other halogenation reactions may be carried out simi-
larly by one ordinarily skilled in the art.
In order to maintain the high selectivity for alcohol
production that is a characteristic of the present invention, it is
important that the added halogen source not be depleted during the
reaction. This may be accomplished by oxidizing the halide and
oxyhalide by-products of the reaction to regenerate gaseous halogen
and recycling them back into the system, or by adding additional
halogen source to the reaction. Otherwise, at the point at which the
added halogen source is depleted, the reaction may continue for a time
and is identified by the formation of the metallic (zero valence) form
of the transition metal portion of the complex (e.g., metallic plati-
num for the Na2PtCl4/Na2PtCl6 complex). It should be emphasized that
in order to carry out chlorination, the platinum complex is required
in a concentration of from about 0.001 mole/liter to about
mole/liter, preferably from about 0.01 mole/liter to about
mole/liter, more preferably from about 0.1 mole/liter to about 0.5
mole/liter. However, in order to carry out the hydrolysis step, the
platinum complex in a concentration of from about 0.01 mole/liter to
about 1 mole/liter is required.
The following examples are illustrative and not intended to
limit the scope of the invention.
Example 1
Commercially available chlorine, methane, Na2PtCl4 and
Na2PtCl6 were used without further purification. The reaction was
performed at 125C for 2 hours in an 8 ml sapphire high pressure
nuclear magnetic resonance ("NMR") tube with 3 g of a D20 solution
containing 1.2 mmol Na2PtCl6 and 0.16 mmol Na2PtCl4 at 72 psi Cl2 and
208~939
392 psi 13CH4. For an illustration of a sapphire NMR tube assembly,
see I. T. Horvath and E. Ponce, "New Valve Design for High Pressure
Sapphire Tubes for NMR Measurements", Review of Scientific Instru-
ments, Vol. 62, No. 4, pp. 1104-1105 (1991). After 2 hours, the high
pressure NMR spectrum showed the selective formation of methanol
(approximately 40%, in the form of CH30D) as evidenced by the peak at
about 49.5 ppm. NMR indicates the formation of trace amounts of C02,
with a peak at about 125 ppm; CH2(0D)2 (approximately 2%), with a peak
at about 85 ppm; CH2Cl2 (approximately 2%) with a peak at about 56
ppm; CH3Cl, (approximately 5%) with a peak at about 26 ppm; and un-
reacted methane (approximately 50%), with a peak at about -4 ppm.
Formation of metallic platinum was not observed upon visual inspection
of the transparent NMR tube under pressure. HCl that is formed as a
by-product during the formation of methanol may be treated with 2 to
recover the chlorine. Methanol may be separated by distillation by
processes known to one having ordinary skill in the art.
Example 2
Commercially available methyl chloride was used without
further purification. Hydrolysis was carried out in a sapphire high
pressure NMR tube with 39 of a D20 solution containing 1.2 mmol
Na2PtCl6 and 0.16 mmole Na2PtCl4 at 50 psi CH3Cl and heated to 125C
for 1 hour. High pressure ~3C NMR spectrum showed the selective
formation of methanol ~in the form of CH30D). The spectrum had the
following characteristics: a peak at about 49.5 ppm, representing
CH30D (approximately 49%); a peak at about 26 ppm for CH3Cl (approxi-
mately 50%).
