FR2515064A1 - Rhodium, iridium or cobalt complexes - with substd. phenanthroline or bi:pyridyl ligand, useful as shift conversion catalyst - Google Patents

Abstract

New complexes of Rh, Ir or Co contain a phenanthroline or bipyridyl ligand having at least one substituent alpha to an N atom. Prefd. ligands are of formulae (I) and (II): (where R is 1-5C alkyl and M is a metal, pref. alkali metal), esp. (I; R = Me, M + Na). The ligand:metal molar ratio may be 1-3:1, esp. 2:1 for Rh and 1:1 for Ir. The complexes may be prepd. by reacting a Rh, Ir or Co salt with the corresp. ligand in an aq. medium. The complexes are useful as homogeneous catalysts for H2 prodn. by low-temp. water gas shift conversion. They are active over a wide pH range (e.g. 6-10 in the case of Ir).

Description

 The present invention relates to the gas to water conversion reaction and more particularly to catalysts intended to promote the production of hydrogen.

The gas to water conversion reaction corresponds to the following equilibrium:

 It is an interesting process for preparing hydrogen from water, or else for increasing the proportion of hydrogen in the synthesis gas which is used in particular in methanation reactions and Fischer syntheses and Tropsch.

 The implementation of this reaction poses certain problems. Indeed, the equilibrium constant of this conversion reaction increases when the temperature decreases; This is why high conversion rates to hydrogen can only be obtained at low temperatures. Under these conditions, it is necessary to have very active catalysts in order to obtain satisfactory conversion rates while maintaining acceptable reaction rates.

Thus, although the reaction is thermodynamically favorable (AG 298 = - 4.76 Koal for
Liquid H2O), it requires high temperatures and pressures to unwind at acceptable speeds in the presence of heterogeneous catalysts.

 Recently, we have started to develop homogeneous catalysts, among which carbonyl metals used under strongly acid or alkaline conditions or else complexes of phosphine and platinum, rhodium or nickel.

 However, none of these catalysts being entirely satisfactory, it is advantageous to propose new ones capable of further improving the performance of the process.

 This is why the present invention provides new complexes useful in particular as catalysts in the conversion of gas to water, characterized in that they comprise a complex of rhodium, iridium or cobalt with, as ligand , a phenanthroline or bipyridine derivative carrying at least one gold substituent of the nitrogen atoms of the ligand and optionally at least one hydrophilic substituent.

 The nature of the hydrophilic substituents is not decisive in the activist of the complexes according to the invention because the essential function of these substituents is to ensure the solubilization of the ligand in the aqueous phase. Among the hydrophilic substituents, mention should be made in particular of sulfonates.

 Likewise, if the presence of the substituents in a atones of nitrogen of the ligand is essential, they can be varied, their function probably being to prevent the formation of a stable planar structure in the chelate which is observed with unsubstituted derivatives of nitrogen.

dans lesquels R représente un radical alcoyle inférieur en C1 à C5 et M représente un cation d'un métal de préférence alcalin tel que le sodium ou le potassium, en particulier la 2,9-diméthyl-4,7-di(p-phényl-sulfonate de sodium)-l,lO-phénanthroline.Among the phenanthroline derivatives which are particularly advantageous to use, mention should be made of the compounds of formula

in which R represents a lower C1 to C5 alkyl radical and M represents a cation of a preferably alkali metal such as sodium or potassium, in particular 2,9-dimethyl-4,7-di (p-phenyl -sodium sulfonate) -l, lO-phenanthroline.

dans lesquels R représente un radical alcoyle inférieur en C1 à C5 et M, comme précédemment, représente le cation d'un métal, en particulier d'un métal alcalin tel que le sodium ou le potassium.Among the bipyridine derivatives which can be used, mention must be made of the compounds of formula

in which R represents a lower C1 to C5 alkyl radical and M, as before, represents the cation of a metal, in particular of an alkali metal such as sodium or potassium.

 The complexes according to the present invention can be prepared by bringing together in a solvent a rhodium, iridium or cobalt salt and the complexing agent derived from phenanthroline or from bipyridine.

 Preferably, these complexes are prepared in situ in an aqueous medium serving as a solvent.

 Of course, it is possible to use as solvent aqueous solutions containing other water-miscible solvents such as alcohols, in variable proportions, for example ethoxyethanol.

 The respective amounts of rhodium salt, iridium or cobalt relative to the amounts of complexing agent can vary to a large extent. However, the ligand / metal ratio exerting a significant influence on the activity of the complex obtained, a ligand / metal molar ratio of between 1 and 3 is generally used, and preferably a ligand / metal ratio close to 2 for the complexes of rhodium and 1 for iridium complexes.

 Unlike the catalysts used in the prior art, it can be seen that the catalysts according to the present invention, in particular the iridium catalysts, are active in a fairly wide range of pH in the vicinity of neutrality, thus the catalysts according to the invention based on iridium have significant activity for pH values between 6 and 10.

 The examples below are intended to illustrate other characteristics and advantages of the present invention.

In the following, the following abbreviations are used - 4,7-di (sodium p-phenylsulfonate) -1,10-phenanthroline
phen-S
(compound of formula I in which R = H and M = Na) - 4,7-di (sodium p-phenylsulfonate) -2,9-dimethyl-
1,10-phenanthroline:
2.9-dmphen-s
(composed of formula I in which R = CH3 and M = Na)
The phen-S compound is not part of the present invention and is used only for comparison.

Example 1
Preparation of iridium chelate and 2,9-dmphen-s
10 micromoles of IrCl313H20 and 10 micromoles of 2,9-dmphen-S are dissolved in 10 ml of sodium phosphate buffer (1M, pH 5.4).

 The mixture thus obtained is heated to 1000C with magnetic stirring (300 rpm), under argon, for 3 hours.

 An aqueous solution of a complex according to the present invention is thus obtained.

 In order to demonstrate the properties of this complex, it is introduced into the container for 15 min. carbon monoxide (purity greater than 99.995%), after which the intake opening is closed.

 The internal volume of the device is 330 ml, the internal pressure is equalized with atmospheric pressure by means of a mercury valve.

 The gas obtained is analyzed by vapor phase chromatography, its composition being determined as a function of time.

 The average production measured is 15 moles of H2 / mole of catalyst hour.

Examples 2 to 11
In order to demonstrate the advantageous properties of the complex according to the present invention, various complexes according to the invention are produced as well as comparison complexes using the process described in Example 1.

 The catalyst tests are carried out at 1000C at pH 5.4, the pressure being 1 atm. CO as described in Example 1.

 In each experiment, the amount of metal used is 10 micromoles.

The results which are collated in Table 1 are given in number of moles. of H2 by number of moles of catalyst: nH2 / not and in number of moles of H2 / number of moles of catalyst. time: nH2 / ncat.h, TANLEAU I ligand Total time nH2 / ncat nH2 / ncat.n
Example Metal salt (molar heating ratio (hour-1)
ligand / metal) (hour) 2 RhCl3.3H2O - 14 0.5 0.04 3 RhCl3.3H2O phen-S (2) 18.5 1.1 0.06 4 RhCl3.3H2O 2.9-dmphen-S ( 1) 16.4 178 10.8 5 RhCl3.3H2O 2.9-dmphen-S (2) 16.4 382 23.3 6 RhCl3.3H2O 2.9-dmphen-S (3) 16.4 224 13, 7 7 IrCl3.3H2O - 18.2 2.6 0.15 8 IrCl3.3H2O phen-S (1) 18.5 1.1 0.06 9 IrCl3.3H2O 2.9-dmphen-S (1) 19 290 15 10 IrCl3.3H2O 2.9-dmphen-S (2) 18.3 172 9.4 11 IrCl3.3H2O 2.9-dmphen-S (3) 18.1 196 10.8
In general, the reaction mixture turns dark blue for the iridium complexes after a few minutes when the carbon monoxide is detected in the container.

 When starting from catalytic systems prepared in situ by mixing RhCl3.3H20 and the desired amount of ligand (examples 3 to 6) the mixture becomes brown, then a black precipitate appears.

 It appears to correspond to highly insoluble products which are perhaps mono- or dihydric complexes, although spectroscopic studies do not very clearly demonstrate the presence of such compounds.

 The catalyst prepared from iridium chloride and an equivalent of 2,9-dimethylphenanthroline in an ethoxyethanol / water mixture (4/1) has proved to be the most effective catalyst with a production of 100 moles of hydrogen per mole of catalyst and per hour operating at 1000C, no precipitation is observed and the system produces up to 1000 moles of hydrogen for the iridium originally present.

It should be noted that the pH has a fairly significant effect on the activity of the catalysts, in particular on rhodium-based catalysts. Indeed, the iridium-based catalysts appear to be relatively insensitive to pH and give satisfactory results in the range of 6 to 10. As the formation of
C02 is accelerated in basic medium and that the formation of H2 is favored by a low pH, the influence of pH for rhodium complexes whose optimum is between 5 and 6 and the effectiveness of these compounds around neutrality are an indication that the oxidation kinetics of carbon monoxide and the release of H2 are very important.

 From the examples in Table 1, it can be seen that the phenanthroline complexes having nitrogen atoms, that is to say in positions 2 and 9, methyl substituents are approximately 300 times more effective than the corresponding non-phenanthroline derivatives. methylated.

 Although the Applicant does not wish to be limited by any scientific explanation, it is believed that the unsubstituted phenanthroline complexes can adopt a square planar configuration, while the substituted phenanthroline derivatives can - orize the mono-chelate complexes or non-planar geometry. The stabilization of a square planar geometry by unsubstituted phenanthroline derivatives may explain the low activity of the corresponding complexes.

 Table 1 also shows that the molar ratio between the metal and the ligand has an important influence on the catalytic activity of the system, both for rhodium and for iridium.

 In the appended figure, which gives the production of hydrogen in mole / mole of catalyst. Hour as a function of the mole of ligand / mole of metal ratio, it appears that the most active catalysts comprise two equivalents of the phenanthroline derivative for rhodium whereas the most efficient catalysts for iridium are obtained by reacting one mole of ligand per mole of metal.

 In the case where a chelating agent is not used, as for example in experiments 1 and 6, a precipitation of metal is observed which is due to the reduction of the starting salt by carbon monoxide.

 The most efficient catalysts are stable under experimental conditions and the activity of these systems is constant for a large number of days.

In addition, the electron spectra of the starting solutions of the reaction mixture, after formation of approximately 100 moles of hydrogen for rhodium, are identical to the starting spectra. Thus, although the comparison with other catalysts for the conversion of gas to water already described is difficult because the operating conditions are in all cases very different, the present catalysts seem to be among the most active systems. In addition, they cover a fairly wide pH band around neutrality, an area in which the previously known catalytic systems seemed to exhibit little activity.

Claims (10)

 1) Complex of rhodium, iridium or cobalt with, as ligand, a phenanthroline or bipyridine derivative carrying at least one substituent in has nitrogen atoms of the ligand.  2) Complex according to claim 1, characterized in that the ligand carries at least one hydrophilic substituent.  3) Complex according to claim 2, characterized in that the hydrophilic substituent is a sulfonate radical.  4) Complex according to one of claims 1 to 3, characterized in that the substituent has nitrogen atoms is an alkyl radical.  5) Complex according to claim 4, characterized in that the substituent at a is a methyl radical.  6) Complex according to claim 2, characterized in that the phenanthroline derivative is 2,9-dimethyl-4,7-di (sodium p-phenyl-sulfonate 1, 10-phenanthroline.  7) Complex according to one of claims 1 to 6, characterized in that the molar ratio between the phenanthroline or bipyridine derivative and the metal is between 1 and 3.  8) Complex according to one of claims 1 to 7, characterized in that the molar ratio between the phenanthroline or bipyridine derivative and the metal is about 2 for rhodium and 1 for iridium.  9) Application of the complexes according to one of claims 1 to 8 as catalysts for the conversion of gas to water.  10) Process for the preparation of complexes according to one of claims 1 to 9, characterized in that a rhodium, iridium or cobalt salt is reacted in the aqueous phase with the ligand.