Process for C.sub.1 -C.sub.4 alkanol production from synthesis gas

This invention concerns a process for making C.sub.1 -C.sub.4 alkanols and particularly ethanol which comprises contacting a mixture of CO and H.sub.2 at a pressure of 30 atm or greater and at a temperature of at least 150.degree. C. with a catalyst system comprising a ruthenium-containing compound and a cobalt-containing compound dispersed in a low melting quaternary phosphonium base with an added N-heterocyclic promoter.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention concerns an improved process for C.sub.1 -C.sub.4 alkanol 
production from synthesis gas under moderate pressure. An improvement in 
the selectivity of alcohol relative to ester is achieved with promoters, 
with C.sub.1 -C.sub.4 alkanols making up to 66 wt % of the total liquid 
product. In addition to improved selectivity for C.sub.1 -C.sub.4 
alkanols, there is improved selectivity for ethanol which comprises up to 
54% of that fraction. 
2. Prior Art 
It has long been known that monofunctional alcohols such as methanol, 
ethanol, etc. can be formed by the reaction of synthesis gas, i.e., a 
mixture of carbon monoxide and hydrogen at elevated pressures of, for 
example, up to 1000 atmospheres, and at temperatures of from about 
200.degree. to 500.degree. C. or more using as a catalyst a mixture of 
copper, chromium and zinc oxides. A wide variety of other catalysts have 
been employed in the reaction of carbon monoxide and hydrogen to yield 
liquid products containing substantial amounts of monofunctional alcohols 
as exemplified by methanol, ethanol, propanol, etc. For example, in U.S. 
Pat. No. 4,013,700 the reaction of carbon monoxide and hydrogen in the 
presence of a quaternary phosphonium cation and a rhodium carbonyl complex 
yields a liquid product having a high methanol content. In U.S. Pat. No. 
4,014,913 where the same reactants are contacted with a solid catalyst 
comprising a combination of rhodium and manganese the product formed 
contains substantial amounts of ethanol and in U.S. Pat. No. 4,197,253 
where the reaction of carbon monoxide and hydrogen is conducted in the 
presence of a rhodium carbonyl complex and a phosphine oxide compound the 
resulting product contains a high concentration of methanol. Likewise, 
when the same reactants are contacted with a rhodium carbonyl complex and 
a copper salt a liquid product containing a substantial amount of methanol 
is formed. 
One serious problem associated with synthesis gas operations in the past 
has been the non-selectivity of the product distribution since high 
activity catalysts generally yield a liquid product containing numerous 
hydrocarbon materials. Thus, complicated recovery schemes are necessary to 
separate the desired product and the overall yield of the valuable organic 
products is low. This is a definite need in the art for a process which 
will produce alkanols, especially ethanol-rich alkanols, with a high 
degree of selectivity from synthesis gas. 
The discovery of a process for making alkanols at moderate pressures, with 
improved selectivity for ethanol by using a unique catalyst system with a 
novel promoter would be an advance in the art. The ethanol, methanol, 
propanol and butanol would be useful as octane enhancers for gasoline 
blending. 
SUMMARY OF THE INVENTION 
This invention concerns a method for making C.sub.1 -C.sub.4 alkanols, 
especially ethanol, which comprises contacting a mixture of CO and H.sub.2 
at a pressure of 30 atm or greater and at a temperature of at least 
150.degree. C. with a catalyst system comprising a ruthenium-containing 
compound and a cobalt-containing compound dispersed in a low melting 
quaternary phosphonium salt with added N-heterocyclic promoters. 
DETAILED DESCRIPTION OF THE INVENTION 
In the narrower and more preferred practice of this invention, C.sub.1 
-C.sub.4 alkanols, especially ethanol, are prepared by contacting a 
mixture of CO and H.sub.2 at a temperature of about 150.degree. to about 
350.degree. C. and at a pressure of 30 atm or greater with a catalyst 
system comprising one or more ruthenium-containing compounds and one or 
more cobalt-containing compounds dispersed in a low melting quaternary 
phosphonium salt with added N-heterocyclic promoters such as 
2,2'-dipyridyl. 
As previously pointed out the catalyst system employed in the practice of 
this invention contains one or more ruthenium-containing compounds and one 
or more cobalt-containing compounds. The ruthenium-containing catalyst as 
well as the cobalt-containing catalyst may be chosen from a wide variety 
of organic or inorganic compounds, complexes, etc., as will be shown and 
illustrated below. It is only necessary that the catalyst precursor 
actually employed contain the metals in any of their ionic states. The 
actual catalytically active species is then believed to comprise ruthenium 
and cobalt in complex combination with the quaternary salt, promoter and 
with carbon monoxide and hydrogen. The most effective catalysis is 
believed to be achieved where ruthenium and cobalt hydrocarbonyl species 
are solubilized in a quaternary phosphonium salt with an N-heterocyclic 
promoter under reaction conditions. 
The ruthenium catalyst precursors may take many different forms. For 
instance, the ruthenium may be added to the reaction mixture in an oxide 
form, as in the case of for example, ruthenium(IV) oxide hydrate, 
anhydrous ruthenium(IV) dioxide and ruthenium(VIII) tetraoxide. 
Alternatively, it may be added as the salt of a mineral acid, as in the 
case of ruthenium(III) chloride hydrate, ruthenium(III) bromide, 
ruthenium(III) triiodide, tricarbonyl ruthenium(II) iodide, anhydrous 
ruthenium(III) chloride and ruthenium nitrate, or as the salt of a 
suitable organic carboxylic acid, for example, ruthenium(III) acetate, 
ruthenium naphthenate, ruthenium valerate and ruthenium complexes with 
carbonyl-containing ligands, such as ruthenium(III) acetylacetonate. The 
ruthenium may also be added to the reaction zone as a carbonyl or 
hydrocarbonyl derivative. Here, suitable examples include triruthenium 
dodecacarbonyl and other hydrocarbonyls such as H.sub.2 Ru.sub.4 
(CO).sub.13 and H.sub.4 Ru.sub.4 (CO).sub.12, and substituted carbonyl 
species such as the tricarbonylruthenium(II) chloride dimer, [Ru(CO).sub.3 
Cl.sub.2 ].sub.2. 
Preferred ruthenium-containing compounds include oxides of ruthenium, 
ruthenium salts of an organic carboxylic acid and ruthenium carbonyl or 
hydrocarbonyl derivatives. Among these, particularly preferred are 
ruthenium(IV) dioxide hydrate, ruthenium(VIII) tetraoxide, anhydrous 
ruthenium(IV) oxide, ruthenium acetate, ruthenium(III) acetylacetonate, 
and triruthenium dodecacarbonyl. 
The cobalt-containing catalyst precursors may take many different forms. 
For instance, the cobalt may be added to the reaction mixture in an oxide 
form, as in the case of, for example, cobalt(II) oxide (CoO) or 
cobalt(II,III) oxide (Co.sub.3 O.sub.4). Alternatively, it may be added as 
the salt of a mineral acid, as in the case of cobalt(II) nitrate, hydrate 
(Co(NO.sub.3).sub.2.6H.sub.2 O), cobalt(II) sulphate, etc., or as the salt 
of a suitable organic carboxylic acid, for example, cobalt(II) formate, 
cobalt(II) acetate, cobalt(II) propionate, cobalt(II) oxalate, cobalt 
naphthenate, as well as cobalt complexes with carbonyl-containing ligands 
as in the case of cobalt(II) acetylacetonate and cobalt(III) 
acetylacetonates, etc. The cobalt may also be added to the reaction zone 
as cobalt carbide, cobalt(II) carbonate and a carbonyl or hydrocarbonyl 
derivative. Here, suitable examples include dicobalt octacarbonyl 
(Co.sub.2 (CO).sub.8), cobalt hydrocarbonyl (HCo(CO).sub.4) and 
substituted carbonyl species such as the triphenylphosphine cobalt 
tricarbonyl dimer, etc. 
Preferred cobalt-containing compounds include oxides of cobalt, cobalt 
salts of organic carboxylic acids and cobalt carbonyl or hydrocarbonyl 
derivatives. Among these, particularly preferred are cobalt(II) 
acetylacetonate, cobalt(III) acetylacetonate, cobalt(II) acetate, 
cobalt(II) propionate, and dicobalt octacarbonyl. 
The ruthenium-containing compound and cobalt-containing compound are, prior 
to their catalytic use in making alkanols, first dispersed in a low 
melting quaternary phosphonium salt. It is interesting to note that the 
ruthenium-containing compound alone, without being dispersed in said salt, 
has little, if any activity in promoting the manufacture of alkanols from 
synthesis gas. 
The quaternary phosphonium salt must be relatively low melting, that is, 
melt at a temperature less than about the temperature of reaction of 
making alkanols. Usually the quaternary compound has a melting point less 
than about 180.degree. C., and most often has a melting point less than 
150.degree. C. 
Suitable quaternary phosphonium salts have the formula: 
##STR1## 
where R.sub.1, R.sub.2, R.sub.3 and R.sub.4 are organic radicals, 
particularly aryl or alkaryl radicals bonded to the phosphorous atom, and 
X is an anionic species. The organic radicals useful in this instance 
include those alkyl radicals having 1 to 20 carbon atoms in a branched or 
linear alkyl chain; they include the methyl, ethyl, n-butyl, iso-butyl, 
octyl, 2-ethylhexyl and dodecyl radicals. Tetraethylphosphonium bromide 
and tetrabutylphosphonium bromide are typical examples presently in 
commercial production. The corresponding quaternary phosphonium acetates, 
hydroxides, nitrates, chromates, tetrafluoroborates and other halides, 
such as the corresponding chlorides, and iodides, are also satisfactory in 
this instance. 
Equally useful are the phosphonium salts containing phosphorus bonded to a 
mixture of alkyl, aryl and alkaryl radicals. Said aryl and alkaryl 
radicals may each contain 6 to 20 carbon atoms. The aryl radical is most 
commonly phenyl. The alkaryl group may comprise phenyl substituted with 
one or more C.sub.1 -C.sub.10 alkyl substituents, bonded to the phosphorus 
atom through the aryl function. 
Illustrative examples of suitable quaternary phosphonium salts include 
tetrabutylphosphonium bromide, heptyltriphenylphosphonium bromide, 
tetrabutylphosphonium iodide, tetrabutylphosphonium chloride, 
tetrabutylphosphonium nitrate, tetrabutylphosphonium chromate, 
tetrabutylphosphonium tetrafluoroborate and tetrabutylphosphonium acetate. 
Table I provides evidence of the effectiveness of the quaternary 
phosphonium salts when in combination with triruthenium dodecacarbonyl. 
The preferred quaternary salts are generally the tetralkylphosphonium salts 
containing alkyl groups having 1-6 carbon atoms, such as methyl, ethyl and 
butyl. Tetrabutylphosphonium salts, such as tetrabutylphosphonium bromide, 
are most preferred for the practice of this invention. 
Preferred tetrabutylphosphonium salts include the bromide, chloride, 
iodide, acetate and chromate salts. 
The promoter which is used in accordance with the process of this invention 
may be any N-heterocyclic promoter which is at least partially soluble in 
the reaction mixture (e.g. has a solubility of at least about 10 ppm in 
the reaction mixture). Illustrative of typical N-heterocyclic compounds of 
this character include pyridine and its derivatives such as 3,5-lutidine, 
2,6-methoxypyridine, 4-dimethylaminopyridine, the collidines and 
2,6-lutidine. Also effective are polycyclic, N-heterocyclics such as 
quinoline, isoquinoline and substituted derivatives thereof such as 
2,2-biquinoline, lepidine, and quinaldine. The preferred heterocyclic 
promoters contain two or more N-heterocyclic atoms per molecule, such as 
2-ethylpyrazine, and include polycyclic N-heterocyclics such as 
2,2'-dipyridyl, 2,3'-dipyridyl, 2,4'-dipyridyl, 2,2'-bipyrimidine, 
2,2',2"-terpyridyl, 2,4,6-tris(2-pyridyl)-S-triazine, 4,4'-dipyridyl, 
4,4'-dimethyl-2,2'-dipyridyl, pyrimidine, 2,2'-dipyridylamine, acridine, 
1,10-phenantroline, 2,2'-bipyrazine, and 2,3-bis(2-pyridyl)pyrazine. 
Generally, in the catalyst system the molar ratio of the ruthenium compound 
to the quaternary phosphonium salt will range from about 1:0.01 to about 
1:100 or more and, preferably, will be from about 1:0.5 to about 1:20. 
The quantity of ruthenium compound and the cobalt compound employed in the 
instant invention is not critical and may vary over a wide range. In 
general, the novel process is desirably conducted in the presence of a 
catalytically effective quantity of the active ruthenium species and of 
the cobalt species which gives the desired product in reasonable yield. 
The reaction proceeds when employing as little as about 1.times.10.sup.-6 
weight percent, and even lesser amounts of ruthenium together with about 
1.times.10.sup.-6 weight percent or less of cobalt, basis the total weight 
of the reaction mixture. The upper concentration is dictated by a variety 
of factors including catalyst cost, partial pressures of carbon monoxide 
and hydrogen, operating temperature, etc. A ruthenium concentration of 
from about 1.times.10.sup.-5 to about 5 weight percent in conjunction with 
a cobalt concentration of from about 1.times.10.sup.-5 to about 5 weight 
percent, based on the total weight of reaction mixture is generally 
desirable in the practice of this invention. The preferred 
ruthenium-to-cobalt atomic ratio is from 10:1 to 1:10. 
The temperature range which can usefully be employed in these syntheses is 
a variable dependent upon other experimental factors, including the 
pressure, and the concentration and choice of the particular species of 
ruthenium catalyst among other things. The range of operability is from 
about 150.degree. C. to 350.degree. C. when superatmospheric pressure of 
syngas are employed. A narrow range of 180.degree. C. to 250.degree. C. 
represents the preferred temperature range. 
Superatmospheric pressures of 30 atm or greater lead to substantial yields 
of alkanols by the process of this invention. A preferred operating range 
is from 130 atm to 600 atm, although pressures above 600 atm also provide 
useful yields of the desired alkanols. 
The relative amounts of carbon monoxide and hydrogen which may be initially 
present in the syngas, i.e., synthesis gas, mixture are variable, and 
these amounts may be varied over a wide range. In general, the mole ratio 
of CO:H.sub.2 is in the range from about 20:1 up to about 1:20, preferably 
from about 5:1 to 1:5, although ratios outside these ranges may also be 
employed. Particularly in continuous operations, but also in batch 
experiments, the carbon monoxide-hydrogen gaseous mixtures may also be 
used in conjunction with up to 50 percent by volume of one or more other 
gases. These other gases may include one or more inert gases such as 
nitrogen, argon, neon and the like, or they may include gases that may, or 
may not, undergo reaction under CO hydrogenation conditions, such as 
carbon dioxide, hydrocarbons such as methane, ethane, propane and the 
like, ethers such as dimethyl ether, methylethyl ether and diethyl ether, 
alkanols such as methanol and acid esters such as methyl acetate. 
Esters of monocarboxylic acids may also be formed during the course of this 
desired alkanol synthesis. Most often these are ester derivatives of 
acetic acid such as methyl acetate, ethyl acetate and propyl acetate, 
which can be conveniently recovered from the reaction mixture. The 
advantage of this process is an improvement in the selectivity of alcohol 
relative to ester achieved by the use of the promoters of this invention. 
With, for example, the Ru.sub.3 (CO).sub.12 --Co.sub.2 (CO).sub.8 
-2,2'-dipyridyl catalyst combination dispersed in Bu.sub.4 PBr, C.sub.1 
-C.sub.4 alkanols may comprise up to 66 wt % of the total liquid product 
(see Example II), with ethanol making up to 53 wt % of that fraction. 
The novel process of this invention can be conducted in a batch, 
semi-continuous or continuous fashion. The catalyst may be initially 
introduced into the reaction zone batchwise, or it may be continuously or 
intermittently introduced into such a zone during the course of the 
synthesis reaction. Operating conditions can be adjusted to optimize the 
formation of the desired ethanol-rich product, and said material may be 
recovered by methods well known in the art, such as distillation, 
fractionation, extraction and the like. A fraction rich in the ruthenium 
and cobalt catalyst components may then be recycled to the reaction zone, 
if desired, and additional products generated. 
The products have been identified in this work by one or more of the 
following analytical procedures, viz, gas-liquid phase chromatograph 
(glc), infrared (ir), mass spectrometry, nuclear magnetic resonance (nmr) 
and elemental analyses, or a combination of these techniques. Analyses 
have, for the most part, been in parts by weight; all temperatures are in 
degrees centigrade and all pressures in atmospheres (atm).