Catalysis of water-gas shift reaction

The reaction of carbon monoxide and water to produce hydrogen and carbon dioxide is catalyzed by a rhodium or iridium component with an iodide promoter.

BACKGROUND OF THE INVENTION 
The present invention relates to the reaction of carbon monoxide and water 
to produce hydrogen and carbon dioxide. In particular, it is directed to 
such process catalyzed by a rhodium or iridium component with an iodide 
promoter. 
A number of heterogeneous catalysis systems are known for use in the 
water-gas shift reaction, employing oxides of such metals as nickel, iron 
and cobalt, at elevated temperatures such as 400.degree. C. The water-gas 
shift reaction is used to produce hydrogen, particularly for use in the 
synthesis of ammonia. 
As described in application of Arnold Hershman, Ser. No. 801,711, filed of 
even date herewith, it was found that the water-gas shift reaction can be 
effectively catalyzed by iodide-promoted rhodium or iridium catalysts at 
relatively low temperatures and pressures, particularly in the absence of 
other materials readily capable of carbonylation. The temperatures 
employed, such as 125.degree. to 225.degree. C., favor the production of 
hydrogen. 
The catalysts have good activity and give good reaction rates at relatively 
mild temperatures. The process has other advantages characteristic of a 
hemogeneous liquid phase catalytic reaction. 
It is known that the water-gas shift reaction may occur to some extent if 
water is present during carbonylation of various substrates, e.g. 
methanol, in the presence of carbonylation catalysts, but this reaction is 
very minor when rhodium or iridium catalysts are used. However 
iodide-promoted catalysts are very effective for the water-gas shift 
reaction when methanol and similar readily carbonylatable substrates are 
absent and substantial amounts of water are present. 
General carbonylation conditions are useful herein (i.e., conditions used 
in carbonylation reactions) with iodide-promoted rhodium or iridium 
catalysts, with regulation of particular parameters to obtain advantages 
and desired results as described herein. Rhodium and iridium carbonylation 
catalysts are taught in Paulik et al U.S. Pat. Nos. 3,772,380 and 
3,769,329, and Craddock et al U.S. Pat. Nos. 3,816,488, 3,816,489, 
3,579,551 and 3,579,552. 
It has now been discovered that concentrations and conditions have a very 
pronounced effect upon the water-gas reaction rates obtainable with the 
iodide-promoted rhodium and iridium catalysts utilized herein. In 
particular, as will be further discussed hereinbelow, the water 
concentrations, and the relationship of the water concentrations to the 
iodide concentration, having a strong effect upon reaction rates. The 
effect of acidity, in terms of the Hammett acidity function, upon the 
reaction rate is also discussed and factors affecting the acidity are 
described and illustrated. 
Substrates which are readily carbonylatabe are substantially absent during 
the present process. Thus methanol and other hydroxyl-containing compounds 
or olefinic hydrocarbons and other materials with olefinic bonds are 
excluded in the present process. 
In accordance with the present invention, carbon monoxide and water are 
reacted at temperatures from about 50.degree. C. to 300.degree. C., 
preferably 125.degree. C. to 225.degree. C., and at partial pressures of 
carbon monoxide from 1 p.s.i.a. to 15,000 p.s.i.a., preferably 5 p.s.i.a. 
to 3,000 p.s.i.a., and more preferably 25 p.s.i.a. to 1,000 p.s.i.a., 
although higher pressures may be employed, in the presence of a catalyst 
system comprised of rhodium-or iridium-containing component, and a 
promoter portion i.e., in iodide. A temperature range of 150.degree. to 
200.degree. C. is particularly suitable, and at pressures say of 100 
p.s.i.a. to 1,000 p.s.i.a. The iodide may be derived from iodine or iodine 
compounds. 
For purposes of the present invention, the catalyst system essentially 
includes a rhodium or iridium component and a halogen component in which 
the halogen is iodine. Generally, the rhodium component of the catalyst 
system of the present invention is believed to be present in the form of a 
coordination compound of rhodium with an iodide component providing at 
least one of the ligands of such coordination compound. In addition to the 
rhodium or iridium and iodide, in the process of the present invention, 
these coordination compounds also generally include carbon monoxide 
ligands thereby forming such compounds or complexes, for example, as 
[Rh(CO).sub.2 I].sub.2 and the like. Other moieties may be present if 
desired. Generally, it is preferred that the catalyst system contain as a 
promoting component, an excess of iodide over that present as ligands in 
the coordination compound. The terms "coordination compound" and 
"coordination complex" used throughout this specification mean a compound 
or complex formed by combination of one or more electronically rich 
molecules or atoms capable of independent existence with one or more 
electronically poor molecules or atoms, each of which may also be capable 
of independent existence. 
The essential rhodium or iridium and iodide component of the catalyst 
system of the present invention may be provided by introducing into the 
reaction zone a coordination compound of rhodium or iridium containing 
iodide ligands or may be provided by introducing into the reaction zone 
separately a rhodium or iridium compound and an iodine compound. Among the 
materials which may be charged to the reaction zone to provide the rhodium 
component of the catalyst system of the present invention are rhodium 
metal, rhodium salts and oxides, organo rhodium compounds, coordination 
compounds of rhodium, and the like. Specific examples of materials capable 
of providing the rhodium constituent of the catalyst system of the present 
invention may be taken from the following non-limiting partial list of 
suitable materials. 
RhCl.sub.3 
RhBr.sub.3 
RhI.sub.3 
RhCl.sub.3 .multidot.3H.sub.2 O 
RhBr.sub.3 .multidot.3H.sub.2 O 
Rh.sub.2 (CO).sub.4 Cl.sub.2 
Rh.sub.2 (CO).sub.4 Br.sub.2 
Rh.sub.2 (CO).sub.4 I.sub.2 
Rh.sub.2 (CO).sub.8 
Rh[(C.sub.6 H.sub.5).sub.3 P].sub.2 (CO)I 
Rh[(C.sub.6 H.sub.5).sub.3 P].sub.2 (CO)Cl 
Rh metal 
Rh(NO.sub.3).sub.3 
RhCl[(C.sub.6 H.sub.5).sub.3 P].sub.2 (CH.sub.3)].sub.2 
Rh(SnCl.sub.3)[(C.sub.6 H.sub.5).sub.3 P].sub.3 
RhCl(CO)[(C.sub.6 H.sub.5).sub.3 As].sub.3 
Rhi(co)[(c.sub.6 h.sub.5).sub.3 sb].sub.2 
[(n-C.sub.4 H.sub.9).sub.4 N][Rh(CO).sub.2 X.sub.2 ] where 
X--Cl,Br.sup.-,I-- 
[n-C.sub.4 H.sub.9).sub.4 As][Rh.sub.2 (CO).sub.2 Y.sub.4 ] where 
Y--Br.sup.-,I-- 
[(n-C.sub.4 H.sub.9).sub.4 P][Rh(CO)I.sub.4 ] 
Rh[(C.sub.6 H.sub.5).sub.3 P].sub.2 (CO)Br 
Rh[n-C.sub.4 H.sub.9).sub.3 P].sub.2 (CO)Br 
Rh[(n-C.sub.4 H.sub.9).sub.3 P](CO)I 
RhBr[(C.sub.6 H.sub.5).sub.3 P].sub.3 
RhI[(C.sub.6 H.sub.5).sub.3 P].sub.3 
RhCl[(C.sub.6 H.sub.5).sub.3 P].sub.3 
RhCl[C.sub.6 H.sub.5).sub.3 P].sub.3 H.sub.2 
[(c.sub.6 h.sub.5)p].sub.3 rh(CO)H 
Rh.sub.2 O.sub.3 
[rh(C.sub.2 H.sub.4).sub.2 Cl].sub.2 
K.sub.4 rh.sub.2 Cl.sub.2 (SnCl.sub.3).sub.4 
K.sub.4 rh-Br.sub.2 (SnBr.sub.3).sub.4 
K.sub.4 rh.sub.2 I.sub.2 (SnI.sub.3).sub.4 
With those materials listed above as capable of providing the rhodium 
component which do not contain an iodine component, it will be necessary 
to introduce in the reaction zone such iodide component. For example, if 
the rhodium component introduced is rhodium metal or Rh.sub.2 O.sub.3, it 
will be necessary to also introduce a halide component such as methyl 
iodide, hydrogen iodide, iodine or the like. 
As noted above, while the halogen component of the catalyst system may be 
in combined form with the rhodium, as for instance, as one or more ligands 
in a coordination compound of rhodium, it generally is preferred to have 
an excess of halogen present in the catalyst system as a promoting 
component. By excess is meant an amount of halogen greater than 2 atoms of 
halogen per atom of rhodium in the catalyst system. This promoting 
component of the catalyst system consists of iodine and/or iodine 
compounds, such a hydrogen iodide, alkyl- or aryl iodide, metal iodide, 
ammonium iodide, phosphonium iodide, arsonium iodide, stibonium iodide and 
the like. The iodide of the promoting component may be the same or 
different from that already present as ligands in the coordination 
compound of rhodium. 
Iodine or iodide compounds are suitable for the promoter portion of the 
catalyst, but those containing iodide are preferred, with hydrogen iodide 
constituting a more preferred member. Accordingly, suitable compounds 
providing the promoter portion of the catalyst system of this invention 
may be selected from the following list of preferred iodide and/or iodine 
containing compounds: 
RI.sub.n where R=any alkyl, alkylene or aryl-group, e.g., Ch.sub.3 I, 
C.sub.6 H.sub.5 I, Ch.sub.3 Ch.sub.2 I, ICH.sub.2 I, etc. (n is 1-3) 
##STR1## 
Where R=any alkyl or aryl-group, e.g., 
##STR2## 
R.sub.4 MI, R.sub.4 MI.sub.3, or R.sub.3 MI.sub.2 where R=hydrogen or any 
alkyl- or aryl-group, M=N, P, As or Sb, eg. NH.sub.4 I, PH.sub.4 I.sub.3, 
PH.sub.3 I.sub.2 (C.sub.6 H.sub.5).sub.3 PI.sub.2, and/or combinations of 
R M and I. 
It is recognized that some moieties of suitable promoter compounds may be 
subject to carbonylation, but in view of the relatively small amounts 
generally involved, such carbonylation will not induly interfere with the 
reaction of carbon monoxide with water which may proceed after more 
readily reacting materials have been consumed. 
Similarly, iridium components of catalyst systems can be charged as iridium 
metal, iridium salts and oxides, organo iridium compounds, coordination 
compounds of iridium, and the like, specific examples being IrCl.sub.3, 
IrBr.sub.3, IrI.sub.3, IrCl.sub.3 .multidot.H.sub.2 O, and the various 
other materials illustrated by substituting Ir for Rh in any of the 
materials in the nonlimiting list of rhodium materials disclosed herein. 
Generally it is preferred that the process of the present invention be 
carried out in an acidic reaction medium, as such appears characcteristic 
of, or to facilitate, conversions involved in the catalysis. The acidity 
is generally such as to be capable of forming alkyl halide from alcohol or 
olefin if such were added to the reaction medium, and particular acidity 
factors are further described herein. 
The catalyst can be formed in situ in the reactor, or be formed separately. 
For instance, a catalyst precursor, e.g., RhCl.sub.3 .multidot.3H.sub.2 O 
or Rh.sub.2 O.sub.3 .multidot.5H.sub.2 O, may be dissolved in a dilute 
aqueous acid solution, e.g., HCl, acetic acid, etc., as solvent. Then the 
solution of the rhodium compound is heated, for example, to 60.degree. 
C.-80.degree. C., or in general at a temperature below the boiling point 
of the solvent, with stirring. A reducing agent such as carbon monoxide is 
bubbled through the said solution and subsequently, the iodine promoter is 
added as described herein. 
Another embodiment of the present invention employs compounds of monovalent 
rhodium or iridium initially, wherein the transformation to active 
catalyst does not involve a change of valence. For example, monovalent 
rhodium salts such as Rh[(C.sub.6 H.sub.5).sub.3 P].sub.3 Cl, [Rh(C.sub.6 
H.sub.5).sub.3 P].sub.3 (CO)Cl, Rh(C.sub.6 H.sub.5).sub.3 P].sub.3 H and 
[Rh(CO).sub.2 Cl].sub.2 are dissolved in a suitable solvent and carbon 
monoxide is subsequently passed through a solution that is preferably 
warmed and stirred. Subsequent addition of a solution of the halogen 
promoter, e.g., alkyl iodide, elemental iodine, aqueous HI, etc., results 
in formaion of an active carbonylation catalyst solution containing the 
necessary rhodium and iodide components. Iridium can be substituted for 
rhodium in any of the illustrative rhodium compounds herein above, and the 
preparation carried out as described. 
In carrying out the reaction in liquid phase, any solvent compatible with 
the catalyst system and not interfering with the reaction may be employed. 
The preferred solvent and liquid reaction medium is a mono-carboxylic acid 
having 2-20 carbon atoms, e.g. acetic, propionic, nonanoic, naphthoic, and 
elaidic acids, including isomeric forms. Other inert solvents can be 
employed, although in general it will be preferred to select solvents to 
provide a homogeneous medium with the water present, either with 
individual solvents, or by use of co-solvents, if appropriate. 
The present process results in the production of hydrogen, along with 
carbon dioxide, and the gaseous hydrogen can readily be recovered as 
off-gases, from batch, continuous or semi-continuous procedures. The 
hydrogen can be separated from the carbon monoxide, carbon dioxide or 
other components of the gas stream, or also for some applications be used 
as a reactant stream without purification. The water-gas shift reaction 
itself can be run to reasonably high conversions, or in stages, to consume 
the carbon monoxide and give a product with fairly low concentrations of 
this component. However, it is preferred to operate with substantial 
carbon monoxide pressures, which mitigates against high conversions. The 
carbon dioxide product is also suitable for some uses, but may for some 
applications simply be removed from the product gas by scrubbing, 
absorption or similar procedures. 
The reaction rate is dependent upon catalyst concentration and temperature. 
Concentrations of the rhodium or iridium compound or the first component 
of the catalyst system in the liquid phase between 10.sup.-6 moles/liter 
and 1 mole/liter, are normally employed, with the preferred range being 
0.001 mole/liter to 0.5 mole/liter. Higher concentrations than those set 
forth herein may, however, be used if desired. Higher temperatures also 
favor higher reaction rates. 
The concentration of the second or promoter component of the catalyst 
system can vary widely but will generally be selected to give good rates 
with the particular catalyst concentration and other conditions, and 
usually being within the range of about 10.sup.-2 moles/liter to 10 
moles/liter based on iodine atom, or more narrowly 0.1 mole/liter to 5 
mole/liter. The preferred amounts will be affected by the particular 
promoter, being influenced by its reactivity and availability of the 
iodine content, as well as by its effect on acidity or other 
characteristics of the reaction medium, but are often about 0.1 mole/liter 
to 2 moles/liter. The relationship of promoter and its acidity, and the 
water concentrations, and their significance is further described 
hereinbelow. 
For the water-gas shift reaction, one mole of carbon monoxide is required 
for each mole of water, but either component can be used in excess. The 
reaction can suitably be conducted with ratios of the reactants suitable 
for good reaction rates, with additional reactants supplied to the 
reaction to replace those consumed. In practice appropriate pressures of 
carbon monoxide may simply be selected to give good rates. However, for 
economic reasons or to have product suitable for particular application, 
it may be appropriate to select conditions to give high conversions of the 
carbon monoxide, which may include use of excess water. On the other hand, 
CO pressure contributes to catalyst stability and may affect reaction 
rate. The water will generally be employed in concentrations suitabe to 
give desired reaction rates. Generally, a substantial amount of water will 
be present ranging up to dilute aqueous solutions containing the catalytic 
components, but adjusted along with other factors as taught herein to give 
desirable reaction rates. 
Procedures employed in the following examples which illustrate the 
invention were carried out in an autoclave reactor with a 1500 ml. 
capacity, with a liquid charge of 500 ml., and an agitator speed of 500 
rpm. In general the procedures were carried out in the same manner for 
comparison purposes. A catalyst slurry was formed by mixing hydrated 
rhodium oxide with concentrated hydriodic acid (57%), and stirring for 10 
minutes under a nitrogen purge, followed by water and acetic acid with 
stirring under the nitrogen purge for an additional 10 minutes. The 
catalyst slurry was then transferred to the autoclave reactor with 
agitation and additional water, acetic acid, HI or other components as 
necessary to attain the concentrations as reported herein. 
The reactor was flushed with CO by pressuring and venting, and charged to 
desired partial pressure of CO, and stirred and heated to desired 
temperature, with a small flow of off-gas. When the desired temperature 
was attained, the off-gas rate, measured by a rotameter, was increased to 
about 1000 ml./minute. The off-gas was sampled by mass spectrometer and a 
chromatographic analyzer. Rate determinations were made when the off-gas 
showed a nearly constant composition, being calculated on the CO.sub.2 
content as analyzed by the chromatographic analyzer. The mass 
spectrometric analyses for CO.sub.2 was generally in good agreement with 
the chromatographic analysis and in most cases the mass spectrometric 
analyses showed the hydrogen and CO.sub.2 content to be substantially 
equivalent. The water concentrations were determined by anaylsis after the 
reaction was stopped in some cases, as well as by measurement of the 
initial charge. The procedures were generally completed in about one hour.

EXAMPLE 1 
Several preparations in accord with the above procedure were carried out at 
varying conditions of temperature, pressure, and concentrations of HI and 
water. Results are reported in Table 1. The various conditions are shown 
to have a significant effect on the rate. In the particular sources used 
in this example, at the lower water conditions, the lower HI 
concentrations have a favorable influence. At a 1.2 molar HI 
concentration, the rate is higher at 22.5 M water than at either 7.5 M or 
30 M. (M as used herein means moles/liter, i.e. molar). Higher 
temperatures enhance the rate. 
TABLE I 
__________________________________________________________________________ 
(Agitator Speed - 500 RPM) 
Total 
Concentration moles/l 
Run Temp. 
Pres. Initial 
No. .degree. C. 
psig 
Rh HI H.sub.2 O 
Other Water-Gas Rate* 
__________________________________________________________________________ 
1 170 300 0.005 
1.2 30 -- 0.032 
2 170 200 0.005 
1.2 22.5 
-- 0.880 
3 170 300 0.005 
1.2 22.5 
-- 0.362 
4 170 450 0.005 
1.2 22.5 
-- 0.445 
5 185 450 0.005 
1.2 22.5 
-- 1.86 
6 170 300 0.005 
1.2 7.5 -- 0.041 
7 170 500 0.005 
1.2 7.5 -- 0.056 
8 170 300 0.005 
0.6 7.5 -- 0.193 
9 170 400 0.005 
0.6 7.5 -- 0.149 
10 170 500 0.005 
0.6 7.5 -- 0.173 
11 170 200 0.005 
0.3 7.5 -- 0.346 
12 170 300 0.005 
0.3 7.5 -- 0.292 
13 170 300 0.005 
0.3 7.5 0.9 M NaI 
0.265 
14 170 300 0.005 
0.3 7.5 0.9 M HCl 
0.360 
15 170 300 0.005 
0 7.5 1.2 M HCl 
0.0027 
__________________________________________________________________________ 
*Moles of CO.sub.2 Produced per Liter of Reactor Solution per Hour 
EXAMPLE 2 
A set of experiments in accord with the above procedure were carried out to 
determine the effects of temperatures and concentrations of HI, water and 
rhodium over ranges as follows 
Rhodium--0.001M to 0.005M 
Hi--0.3m to 0.9M 
Water--7M to 23M 
Temperature--175.degree. C. to 185.degree. C. 
The initial design of the experiments is set forth in Table 2, and the 
experiments as carried out, with ssome modifications, are reprted in Table 
3. The water-gas rate goes through a maximum with changing water 
concentration in each of the sets where other reaction variables were held 
constant. Relatively low rates occurred at 3M and 23M water concentration. 
At a 35M water concentration under the described conditions, there was 
apparently no significant water-gas reaction. The effects of water 
concentration under the described conditions are illustrated in FIG. 1. 
The water-gas reaction appears to be accelerated by higher temperatures at 
all water levels, as indicated by the results in Table 3, and also higher 
rhodium concentrations increase the rate. 
TABLE 2 
______________________________________ 
Water-Gas Reaction - Statistical Design of Experiment 
(Total Pressure = 400 psig; Agitator Speed = 500 RPM) 
Rhod- 
Run, ium HI Temp. Initial Water Levels 
DE# Level Level Level (DE#- .1 to .5) 
______________________________________ 
1.1-1.5 
-1 -1 +1 -2 -1 0 +1 +2 
2.3 +1 -1 -1 0 
3.1-3.5 
-1 +1 -1 -2 -1 0 +1 +2 
4.3 +1 +1 +1 0 
5.3 -1 -1 -1 0 
6.1-6.5 
+1 -1 +1 -2 -1 0 +1 +2 
7.1-7.5 
-1 +1 +1 -2 -1 0 +1 +2 
8.3 +1 +1 -1 
9.3 0 0 0 0 
______________________________________ 
Rhodium Concentrations at -1, 0, +1 levels: - 1 = 0.001M, 0 = 0.003M, +1 
0.005M 
HI Concentrations at -1, 0, +1 levels: -1 = 0.3M, 0 = 0.6M, +1 = 0.9M 
Temperature at -1. 0, +1 levels: -1 = 175.degree., 0 = 180.degree. C., +1 
= 185.degree. C. 
Initial Water Concentrations at -2, -1, 0, +1, +2 levels: -2 = 7M, -1 = 
11M, 0 = 15M, 0 = 19m, +2 = 23M 
TABLE 3 
__________________________________________________________________________ 
(Agitator Speed = 500 RPM, Total Pressure = 400 psig) 
Rhodium 
HI Water 
Hammett Acidity 
Exp. 
Conc. 
Conc. 
Temp., 
Conc. 
Function(-H.sub.0) of 
Water-Gas Rate 
DE# 
moles/1 
moles/1 
.degree. C. 
moles/1.sup.1 
Reactor Solution.sup.2 
moles/1-hr.sup.3 
__________________________________________________________________________ 
1.0 
0.001 
0.3 185 3 +1.18 0.0036 
1.1 
0.001 
0.3 185 6.9.sup.4 
+0.42 0.0935 
1.2 
0.001 
0.3 185 10.8.sup.5 
-0.19 0.163 
1.3 
0.001 
0.3 185 15 -0.55 0.021 
1.4 
0.001 
0.3 185 19 -0.81 0.0085 
1.5 
0.001 
0.3 185 23 -0.96 0.0047 
2.3 
0.005 
0.3 175 15 -0.50 0.028 
3.1 
0.001 
0.9 175 7 +0.88 0.0143 
3.2 
0.001 
0.9 175 10.8.sup.5 
+0.52 0.152 
3.3 
0.001 
0.9 175 14.8.sup.6 
+0.26 0.305 
3.4 
0.001 
0.9 175 19 +0.06 0.0875 
3.5 
0.001 
0.9 175 23 -0.06 0.0349 
4.3 
0.005 
0.9 185 14.0.sup.6 
+0.31 1.47 
4.3A 
0.005 
0.9 185 15.0.sup.7 
+0.26 2.03 
5.3 
0.001 
0.3 175 15 -0.55 0.0054 
6.0 
0.005 
0.3 185 3 +1.18 0.0203 
6.1 
0.005 
0.3 185 6.6.sup.4 
+0.47 0.376 
6.2 
0.005 
0.3 185 10.1.sup.5 
-0.12 0.753 
6.3 
0.005 
0.3 185 14.9.sup.6 
-0.54 0.0844 
6.4 
0.005 
0.3 185 19 -0.81 0.0188 
7.1 
0.001 
0.9 185 7 -0.88 0.0255 
7.2 
0.001 
0.9 185 10.8.sup.5 
+0.52 0.2011 
7.3 
0.001 
0.9 185 14.5.sup.6 
+0.28 0.546 
7.4 
0.001 
0.9 185 18.8.sup.8 
+0.07 0.290 
7.5 
0.001 
0.9 185 22.9.sup.9 
-0.06 0.0704 
8.3 
0.005 
0.9 175 13.9.sup.6 
+0.32 1.18 
9.3 
0.003 
0.6 180.5 
14.6.sup.6 
-0.01 0.440 
__________________________________________________________________________ 
1. Water concentration determined by analysis of the reactor solution 
after stopping the reaction. 
2. Acidity function value = -Ho) Determined by extrapolation from the 
values measured at 0.1M, 0.36M and 0.87M HI concentrations, by the 
procedure of M. A. Paul and F. A. Long, Chem. Reviews Vol. 57, page 1, 
1957. 
3. Moles of CO.sub.2 formed per liter of reactor solution per hour. 
4. Initial H.sub.2 O concentration = 7M 
5. Initial H.sub.2 O concentration = 11M 
6. Initial H.sub.2 O concentration = 15M 
7. Initial H.sub.2 O concentration = 16.7M 
8. Initial H.sub.2 O concentration = 19M 
9. Initial H.sub.2 O concentration = 23M 
TABLE 4 
__________________________________________________________________________ 
Rhodium-Catalyzed Water-Gas Reaction - Effects of Additives 
(Rh)= 0.001M. Temp.=185.degree. C., Total Pressure= 400 psig. Agitator 
speed= 500 RPM) 
Hammett Acidity 
Function -H.sub.0) of 
Water-Gas Rate 
H.sub.2 O 
HI NAI 
HCl 
Other Reactor Solution 
moles/1-hr.sup.2 
__________________________________________________________________________ 
6.9 
0.3 
-- -- -- +0.2.sup.3 
0.0935 
7.0 
0.9 
-- -- -- +0.88.sup.3 
0.0255 
6.9 
0.3 
0.6 
-- -- +0.58.sup.4 
0.0755 
6.9 
0.3 
-- 0.6 
-- +1.14.sup.4 
0.101 
15.0 
0.3 
-- -- -- -0.55.sup.3 
0.021 
14.5 
0.9 
-- -- -- +0.28.sup.3 
0.546 
14.9 
0.3 
0.6 
-- -- -0.02.sup.4 
0.152 
15.0 
0.3 
-- 0.6 
-- +0.18.sup.4 
0.069 
15.0 
0.3 
-- -- 0.6M KCl -- 0.0049 
15.0 
0.3 
-- -- 0.072M FeI.sub.2 
-- 0.037 
14.9 
0.3 
-- -- 0.145M NiI.sub.2 
-- 0.051 
15.0 
0.3 
-- -- 0.145M Nickel(ous) 
-- 0.0018 
Acetate 
15.0 
0.3 
-- -- 0.6M H.sub.3 PO.sub.2 
-- 0.050.sup.5 
14.9 
0.3 
-- -- 0.3M Na.sub.2 SO.sub.4 
-- 0.110.sup.5 
23.0 
0.3 
-- -- -- -0.96.sup.3 
0.0047 
22.9 
0.9 
-- -- -- -0.06.sup.3 
0.0704 
23.9 
0.3 
0.6 
-- -- -0.65.sup.4 
0.007 
23.0 
0.3 
-- 0.6 
-- -0.01.sup.4 
0.0101 
__________________________________________________________________________ 
1. Water concentration determined by analysis of the reactor solution 
after stopping the reaction. 
2. Moles of CO.sub.2 formed per liter of reactor solution per hour. 
3. Acidity function values (-H.sub.0) determined by extrapolation from th 
value measured at 0.1M, 0.36M and 0.87M HI concentrations. 
4. Acidity function values (-H.sub.0) measured for solutions of these 
concentrations of water, HI, NaI or HCl, ACOH. (acetic acid) 
5. Rate was not steady. 
EXAMPLE 3 
In order to appraise the effects of total iodide and acidity, several 
preparations were carried out using the same general procedure as above, 
but with sodium iodide and hydrochloric acid in addition to HI. The 
results, along with runs using several other additives, are reported in 
Table 4. The order of the water gas rates was as follows: 
TABLE 5 
__________________________________________________________________________ 
Water 
Conc. 
Relative Water-Gas Rates 
__________________________________________________________________________ 
7M (0.3M HI + 0.6M HCl)&gt;0.3M HI&gt;(0.3M HI + 0.6M NaI)&gt;0.9M HI 
15M 0.9M HI&gt;(0.3M HI + 0.6M NaI)&gt;(0.3M HI + 0.6M HCl)&gt;0.3M HI 
23M 0.9M HI&gt;(0.3M HI + 0.6M HCl)&gt;(0.3M HI + 0.6M NaI)&gt;0.3M 
__________________________________________________________________________ 
HI 
A possible mechanism of the water-gas reaction involves oxidation of a 
rhodium complex by HI with production of hydrogen, followed by reduction 
of the complex by water and carbon monoxide to produce carbon dioxide and 
regenerate HI. Such reactions can be illustrated: 
EQU Rh(CO).sub.2 I.sub.2.sup.- +2HI.fwdarw.Rh(CO)I.sub.4.sup.- +CO+H.sub.2 
EQU rh(CO)I.sub.4.sup.- +H.sub.2 O+2 CO.fwdarw.Rh(CO).sub.2 I.sub.2.sup.- 
+CO.sub.2 + 2HI 
the net result of such reaction in which Rh(I) is oxidized to Rh(III) and 
reduced back to Rh(I) is the water gas reaction: 
EQU H.sub.2 O+CO.fwdarw.H.sub.2 + CO.sub.2 
from experiments in which the valence state of rhodium complexes was 
measured under carbon monoxide in water-HI-acetic acid solutions it was 
found that relative amounts of Rh(III) complex increase with increasing 
HI, while relative amounts of Rh(I) are higher at higher water levels. 
This indicates that under water-gas reaction conditions, the oxidation 
step would be enhanced by higher HI or retarded by higher water 
concentrations, while the reduction step would be retarded by higher HI 
but enhanced by higher water levels. 
It can be seen from FIG. 1, and other results reported herein, high water 
concentration can have a very marked effect upon the water-gas rate. The 
rates in FIG. 1 are on a log scale, and the maxima are in some cases 100 
fold or more greater than minimum values for the same conditions except 
for the different water concentrations. Ordinarily it will be desirable to 
operate in the range of about 3 or 10 to about 25 or so moles/liter of 
water, although the ranges can be extended somewhat by use of higher 
temperatures, higher catalyst concentration, etc., or affected by the 
presence of other components or the form of the iodide promoter. A more 
prescribed range is 5 or 10 to 25 moles/liter of water. In some 
circumstances it may be appropriate to use up to 30 or more moles/liter of 
water. 
With given concentrations and forms of the iodide promoter e.g. HI, and 
other conditions, it will be desirable to operate at a water concentration 
about that to provide the maximum water gas rate, or within about a 5 
molar range on each side of such concentration. Such concentrations can be 
established by initial charge, or established or maintained by addition of 
components during the reaction with adjustment of addition rates as 
appropriate. 
While the relationship of HI and water influences the illustrated oxidation 
and reduction steps and the overall water-gas rate, the steps are also 
influenced by acidity which can be affected by additional components if 
present. Thus experiments have shown that increasing acidity promotes the 
oxidation of the rhodium complex, while declining acidity promotes the 
reduction of the rhodium complex. Thus the acidity will desirably be such 
as to permit both steps to occur at reasonable rates, with the slower 
being more determinative of the overall rate and more susceptible to 
promotion by changes in acidity to affect the overall water-gas rate. 
Since the present reaction media generally involve non-aqueous solvents 
along with water, the pH of the solution is not a valid measure of its 
true acidity. The neutral Hammett acidity function [-H.sub.0 ] was used to 
correlate the acidity of the solution with the water-gas rate. In either 
aqueous or non-aqueous solutions, the -H.sub.0 is determined 
colorimetrically by the degree of association of the acidic species with 
an organic base. H.sub.0 has the same relationship to effective acidity 
(h.sub.0) as pH has to hydrogen ion concentration [H.sup.+ ] in dilute 
aqueous solution. For the reaction media employed herein, the relationship 
of the Hammett acidity fraction and the "effective acid concentration" can 
be expressed 
EQU -H.sub.0 =log h.sub.0 
where: 
-H.sub.0 =Hammett acidity function 
h.sub.0 ="effective acid concentration" 
For HI concentrations in the range of 0.1M to 0.9M in varying water-acetic 
acid solutions, the acidity function was found to be very high at low 
water concentrations and to decrease rapidly with increasing water 
concentrations, in the manner usually characteristic of the Hammett 
acidity function. The -H.sub.0 function varies over about 2 units in the 
range of about 3M to 23M, so the effective acid concentration (h.sub.0) 
varies approximately 100 fold over the range. Thus if the water-gas rate 
is influenced by effective acidity, small changes in water concentration 
would greatly affect the water-gas rate. The relationship between the 
Hammett acidity function and the water-gas rate is illustrated in FIG. 2 
where the Hammett function is plotted against the water-gas rate for a 
number of reaction media of varying water and HI or other iodide component 
content (in acetic acid). The values at the lower and higher acidity 
levels of the illustrated range fall in nearly straight lines. The maximum 
rate occurs at a Hammett acidity function of approximately +0.2. At less 
acidic-values, there is a line of positive slope indicating that 
increasing acidity promotes the reaction. At more acidic values, there is 
a line of negative slope indicating that increasing acidity retards the 
reaction. Also, the figure shows the general significance of effective 
acid concentration to the rate and the usual desirability of staying 
within Hammett Acidity Function ranges of about -1 to about 1.2, for 
faster rates in the range of about -0.4 to +0.8. The presence of other 
components, such as HCl, can cause some variation in the results, as 
illustrated in FIG. 2. In such event Hammett acidity function range based 
on only the HI and water present in the reaction medium may be relevant. 
The significance of the effective acidity is further illustrated in FIG. 3 
in which the log of the water-gas rate is plotted against the Hammett 
acidity function for different temperatures and rhodium concentrations. 
The figure shows the expected increase in rates with temperature and 
rhodium concentration, but the increase is greater at lower acidity, 
causing a shift in the maximum rate to a lower acidity value. However, it 
is still desirable to operate at appropriate acidity to give good rates, 
and this will generally be in the range of Hammett acidity of about -1. to 
about +1.2 or so, recognizing that even within the range it will be 
desirable to operate at values near those producing the maximum rate. In 
the event of anomalies due to the presence of other, or different 
components or conditions than those illustrated, it will be appropriate to 
operate at Hammett acidity functions of the reaction media which appear to 
give water-gas rates near the maximum, or more broadly sufficiently near 
that for the maximum rate to maintain rates at lease one-tenth of the 
maximum rate. Water, iodide or other components can be added or removed as 
suitable for maintaining proper acidity. 
While the Hammett acidity is significant to the reaction rates, it may not 
be necessary to actually determine such acidity in order to conduct the 
reaction at proper acidity, as, for example, optimum concentrations of 
water for particular HI concentrations can be determined empirically for a 
Rh catalyst in an HI-water system, and the Hammett acidity will be a 
function of these components. The present invention is especially 
concerned with the discovery of the very marked effect of the water 
concentration upon the water-gas reaction rate. However the rate is also 
influenced by catalyst concentration, temperature, carbon monoxide 
pressure and similar factors, and these can be adjusted as taught herein, 
along with the water concentration and related factors, to give rates 
suited to desired production processes. It may be desirable to use rhodium 
concentrations well above those illustrated herein if necessary to obtain 
suitable rates. Since rhodium is a scarce and expensive material, it may 
be preferable to regulate the amount of iodide promoter, using for example 
at least about 0.3 molar amounts, or at least 0.5 molar, or possibly more 
than 1 molar. It will be recognized that the attainable rates will be 
considered along with conversions, selectivity, and various economic 
expense factors in determining the most appropriate conditions for 
conducting the process. Many of the illustrative procedures herein use 
rhodium or iridium concentrations in the range of 0.001M to 0.005M, or 
more broadly 0.0005M to 0.01M, and such ranges may be found most suitable 
for general application. The HI promoter may often be used in the range of 
about 0.1M to 1M, but broader ranges, e.g. 0.05M to about 5M or higher may 
be used. Preferred temperatures for use along with the foregoing ranges 
are generally about 150.degree. C. to 200.degree. C. 
In accord with the general procedure described hereinabove and utilized in 
the above Examples, the water-gas shift reaction was conducted with 
iridium as catalyst, that is substituting iridium for rhodium. Results are 
reported in Table 6. 
TABLE 6 
__________________________________________________________________________ 
Iridium Catalyzed Water-Gas Reaction 
(Agitation Speed = 750 rpm) 
CO.sub.2 Formation Rate, 
Concentration, Moles/l 
Moles/1-hr 
Temp. 500 250 200 
.degree. C. 
Ir HI 
H.sub.2 O 
Other 
Ho psig 
psig 
psig 
__________________________________________________________________________ 
185.degree. 
0.001 
0.1 
3 -- +0.14 
0 -- 0 
" " " 7 -- -0.49 
0.0125 
-- -- 
" " " 11 -- -0.99 
0.0152 
-- 0.0137 
" " " 15 -- -1.38 
0.00350 
-- (0.011).sup.(2) 
" " " 20.1 
-- 0.00381 
-- 0.00995 
" " " 23 -- 0.00633 
-- 0.000072 
" " " 9 -- -0.75 
0.0248 
-- 0.00765 
" " " 5 -- -0.18 
&lt;0.0001 
-- -- 
200.degree. 
" " 7 -- -0.49 
0.0073 
&lt;0.001 
-- 
" " " 11 -- -0.99 
(0.0465).sup.(1) 
0.0304 
-- 
190.degree. 
" 0.3 
9 -- +0.03 
0.00563 
-- 0.00171 
185.degree. 
" 0.3 
3 -- +1.18 
0 -- -- 
" " " 7 -- +0.38 
&lt;0.001 
-- -- 
" " " 11 -- -0.14 
0.0473 
0.0342.sup.(3) 
0.012 
" " " 15 -- -0.50 
0.0184 
-- 0.0187 
" " " 11 -- -0.14 
0.0108 
-- 0.00318 
" " " 15 -- -0.50 
0.000579 
-- 0.00056 
" " " 7 -- +0.38 
0.00121 
-- 0 
" " " 13 -- -0.33 
0.00779 
-- 0.00690 
185.degree. 
0.001 
0.1 
6 -- -0.33 
0.000264 
-- -- 
185.degree. 
0.001 
0.3 
19 -- -0.73 
0.00406.sup.(4) 
-- 0.00130 
" " " 23 -- -0.88 
0.00394 
-- 0.00825 
" " " 15 -- -0.50 
0.00179 
-- 0.00588 
185.degree. 
0.001 
0.9 
15 -- +0.26 
0.0115 
-- 0.00276 
" " " 11 -- +0.54 
0.000184 
-- 0 
" " " 19 -- +0.06 
0.00162 
-- &lt;0.0001 
185.degree. 
0.001 
0.3 
14.9 
0.6 0.00 0.0323 
-- 0.03 15 
M Na I 
" " " 11 " .about.10.3 
0.0169 
-- 0.0104 
" " " 7 " +0.58 
0.00035 
-- &lt;0.0001 
" " " 19 " .about.-0.38 
0.00421 
-- 0.0139 
__________________________________________________________________________ 
The water concentration had a strong influence on the rate, as illustrated 
in FIG. 4 in which the water-gas rate is plotted against water 
concentration. The Hammett acidity function is also significant as 
illustrated in FIG. 5 where the log of the water-gas rate is plotted 
against it. While there is some variance from the results with rhodium, 
and the rates are in general lower, some of the same considerations apply. 
Thus desirable water concentration will generally lie in the ranges 
discussed above. In particular, the water concentration will usually be at 
least about 5 molar with iridium, as well as with rhodium, and it will be 
desirable to operate at values of water concentrations within a 5 molar 
range or so on each side of those to give the peak rate, and to operate at 
Hammett acidity functions approaching those where maximum rates are found, 
and generally approximately in the ranges described with respect to 
rhodium above, and the ranges and other parameters herein are to be 
understood as generally applicable to both rhodium and iridium catalysts 
unless specified otherwise. Aside from whether the process is conducted 
within particular ranges or other parameters, it will be desirable to 
conduct the reaction with water concentrations and acidity appropriate for 
acceptable reaction rates. 
In the performance of the process in accord with the present invention it 
will be recognized that water is consumed and concentrations of other 
components may change during the course of the reaction. In practicing the 
invention water can be added as consumed, and other components can be 
added or removed to maintain desired reaction conditions, and in 
particular to maintain water concentrations and acidity levels near those 
producing maximum rates, or within desired ranges thereof, as taught 
herein. 
The illustrative procedures herein indicate the effect of various reaction 
parameters, concentrations and materials upon the reaction rate and 
results. The effects thus demonstrated can be utilized in the 
determination of conditions desirable for production processes, for 
example, by selecting catalyst concentrations and temperatures sufficient, 
in combination with water concentration, acidity, etc. in ranges described 
herein, to achieve desirable production rates, such as better than 5 or 6 
moles/liter/hour of catalyst-containing solution. If necessary to achieve 
such rates, rhodium or iridium concentrations can be raised as high as 
0.01 or more moles/liter, temperatures as high as 200.degree. C. or 
higher, and the iodide promoter as high as 0.5 moles/liter or higher, even 
over 1 mole/liter, but controlled in conjunction with the water 
concentration maintaining conditions for good reactivity as taught herein.