Alkanolamines, such as ethanolamines or propanolamines, have been prepared for many years by reaction of ammonia with ethylene oxide or propylene oxide and the crude mixture of alkanolamines thus obtained is subjected to fractionation by distillation, so as to isolate and recover a pure alkanolamine. The products thus obtained by this method may be initially colourless or even already slightly coloured and may generally, after a storage time of several days to several weeks, gradually become coloured until colours of between yellow and brown are achieved. External factors can accelerate this colouration, such as light or high temperatures. This phenomenon of the colouration of alkanolamines has been known for a long time. It has not been possible to clearly identify or explain the products responsible for this colouration or the mechanisms which result in this colouration. This colouration phenomenon is, for example, reported in"SRI International, Process Economics Program Report", No. 193 (January 1993), pages 6-9 to 6-10.

Alkanolamines, such as ethanolamines and in particular triethanolamine (TEA), are used in numerous fields, for example in the cosmetics industry, in particular soaps, detergents and shampoos, or alternatively dispersing agents and emulsifying agents.

Due to these fields of application, these products are required to meet increasingly strict demands, in particular with the aim of avoiding or reducing these colouration phenomena. Alkanolamines are thus required which have an increasingly high colour stability over time and in particular at high temperatures.

Numerous solutions have already been provided to date but none of them appears to be advantageous in terms of cost, of ease of use, of level of purity, of degree of toxicology and in particular of thermal stability over time.

Thus, United States Patent US 3 819 710 provides a process which consists in subjecting ethanolamines to a catalytic hydrogenation treatment which comprises bringing the ethanolamines into contact with hydrogen in the presence of a catalyst chosen from platinum, palladium and ruthenium and preferably Raney nickel.

United States Patent US 6 291 715 also provides a catalytic hydrogenation treatment of alkanolamines in the presence of a supported catalyst comprising a metal chosen from Re, Ru, Rh, Pd, Os, Ir, Pt and/or Ag and a support chosen from active charcoal, a-alumina, zirconium dioxide and titanium dioxide.

However, it has been observed that none of the catalytic hydrogenation treatments in particular disclosed above in the United States patents can result in an alkanolamine having a high colour stability over time, in particular at high temperatures.

Consequently, great efforts have been devoted to trying to find a treatment which can make it possible both to improve the colour of alkanolamines and, in particular, to improve the stability of the colour of alkanolamines over time, in particular at high temperatures.

Furthermore, N. Das, D. Tichit, R. Durand, P Graffin and B. Coq have shown, in "Catalysis Letters", Vol. 71, No. 3-4 (2001), pages 181 to 185, that it is possible to carry out the synthesis of 4-methyl-2-pentanone by reaction of acetone with hydrogen in the presence of a catalyst based on Pd dispersed over a mixed metal oxide support formed by calcination of a layered double hydroxide (LDH), in particular of a synthetic hydrotalcite.

D. Tichit, B. Coq, S. Cerneaux and R. Durand have also shown, in"Catalysis Today", Vol. 75 (2002), pages 197 to 202, that it is possible to carry out the synthesis of 2-methyl-3-phenylpropanal by reaction of benzaldehyde with propanal in the presence of a catalyst based on Pd dispersed over a mixed metal oxide support formed by calcination of a layered double hydroxide (LDH), in particular of a synthetic hydrotalcite.

In"Catalysis Today", Vol. 11 (1991), pages 173 to 301, it has also been shown that anionic clays of hydrotalcite type, also known under the term of layered double hydroxides (LDH), can be used as catalyst precursors in numerous fields, for example

in basic catalyses for the polymerization of epoxides or for the aldol condensation intended to manufacture aldehydes-alcohols, in the reforming of hydrocarbons with water, in hydrogenation reactions for producing methane, methanol, higher alcohols, paraffins and olefins from syngas, in the hydrogenation of nitrobenzene, in oxidation reactions or in catalyst supports of Ziegler-Natta type.

The present invention relates first of all to a process for the treatment of an alkanolamine having an improved colour stability over time, in particular at high temperatures, comprising an operation in which the alkanolamine is brought into contact with hydrogen in the presence of a catalyst, characterized in that the catalyst comprises an active catalytic component comprising at least one metal Me chosen from Ni, Cu, Ru, Rh, Pd, Ag, Re, Os, Ir and Pt and a solid support component formed by heat treatment of a solid precursor (SP) based on layered double hydroxide (LDH) comprising at least one divalent metal cation and at least one trivalent metal cation.

Depending on the way in which the catalyst can be prepared, the active catalytic component and the support component can be positioned differently with respect to one another. In particular, the catalytic component can either be dispersed over the support component or can be distributed in the support component, in particular in a relatively uniform way. These various arrangements depend essentially on the way in which the catalytic component is inserted into the support component, in particular before or after the heat treatment of the solid precursor (SP) based on layered double hydroxide (LDH).

In a first alternative form, the preparation of the catalyst can comprise the insertion of the catalytic component into the support component after the heat treatment of the solid precursor (SP). Thus, the process for the preparation of the catalyst can comprise first the heat treatment of the solid precursor (SP) based on LDH, so as to form the support component. Subsequently, it can comprise an incorporation of a metal compound based on the metal Me of the catalytic component into the support component, in particular by impregnation, for example in the liquid or dry phase. The incorporation can be followed by an intermediate heat treatment and, preferably, by a reduction of the metal Me, in particular up to an oxidation state of zero, it being possible for the reduction to be carried out before or during the process for the treatment of the alkanolamine. Thus, in this catalyst, the active catalytic component is found to be dispersed over the support component.

In another alternative form, the preparation of the catalyst can comprise the insertion of the catalytic component into the solid precursor (SP) based on LDH before the heat treatment of the latter. Thus, the process for the preparation of the catalyst can comprise the insertion of a metal compound based on the metal Me of the catalytic component into the solid precursor (SP), for example by coprecipitation or by anion exchange, so as to form a solid precursor (SP) doped by a metal compound of the metal Me, comprising in particular the metal compound of the metal Me inserted into the LDH, for example in the form of a di-or trivalent metal cation of the metal Me (Me2+ or Me3+). The process can subsequently comprise the heat treatment of the solid precursor (SP) thus doped. The heat treatment is followed, preferably, by a reduction of the metal Me, in particular up to an oxidation state of zero, it being possible for the reduction to be carried out before or during the process for the treatment of the alkanolamine. Thus, in this catalyst, the active catalytic component is found to be distributed in the support component, in particular in a relatively uniform way.

A layered double hydroxide or LDH (in French"hydroxyde double lamellaire") is a well known solid hydroxide belonging to a general class of layered metal hydroxides.

The layered double hydroxide is generally a solid which exists in the form of lamellae (sheets or layers) and which has a layered double structure (in German "doppelschichtstrukturen") similar to that of brucite (Mg (OH) 2). A layered double hydroxide is generally characterized in that it comprises a divalent metal cation which is partially substituted by a trivalent metal cation. It should also be remembered that the term"double"in LDH means that the compound comprises metal cations at two different oxidation states, in this instance the oxidation states +2 and +3, and that it can additionally comprise at least one or, preferably, at least two different metals (see, for example, "Encyclopedia of Inorganic Chemistry"by R. Bruce King, published by John Wiley & Sons Ltd (1994), Vol. 3, page 1564).

The layered double hydroxide is also known as compound or hydroxide of "hydrotalcite type"due to the fact that the most well known compound of this family is natural hydrotalcite, which corresponds approximately to the general formula Mg6Al2 (OH) s6CO3 4H20. The layered double hydroxide is thus generally known as metal hydroxide with a layered structure of hydrotalcite type. The layered double hydroxide is also sometimes called"anionic clay", in which the metal hydroxide with a layered structure is positively charged and comprises anions, often called

"compensating anions", and optionally water molecules. These anions and water molecules are generally positioned in the interstitial spaces between the layers of the hydroxide and are capable of being displaced and optionally exchanged.

The solid precursor (SP) can be a layered double hydroxide (also called metal hydroxide of hydrotalcite type or anionic clay) corresponding to the general formula: [MN. (OH) 2] A-mH20 (1) in which M and N represent at least two identical or different metals, M2+ represents at least one divalent metal cation of the metal (s) M, N3+ represents at least one trivalent metal cation of the metal (s) N, A represents at least one inorganic or organic anion, generally called"compensating anion", b is a number equal to x, x is a number having a value ranging from 0.1 to 0.5, n represents the charge of the anion A, preferably equal to 1,2 or 3, and m is a number having a value of less than or equal to 1, preferably ranging from 0 to 1.

The number x can be any number ranging from 0.15 to 0.44, preferably from 0.17 to 0.41, in particular from 0.20 to 0.35, especially from 0.25 to 0.33.

The metal M can be at least one metal chosen from Mg, Fe, Co, Ni, Cu and Zn, preferably from Mg, Co, Ni and Zn. The metal N can be at least one metal chosen from Al, V, Cr, Mn, Fe, Co, Ga, Y and In, preferably from Al, Cr, Mn, Fe and Co.

A solid precursor (SP) doped with a metal compound of the metal Me, prepared in particular by insertion of a metal compound of the metal Me into the solid precursor (SP) based on LDH, can correspond to a formula identical to the general formula (1) in which in particular the metals M and N can be metals chosen from those mentioned above and at least one of the two metals M and N can, in addition, be at least one metal Me chosen, on the one hand, from Pd, Pt and Cu, preferably from Pd and Pt (when the metal Me is one of the metals M), and, on the other hand, from Ru, Rh, Os and Ir, preferably from Rh and Ir (when the metal Me is one of the metals N).

The layered double hydroxide has a"compensating anion"A, such as the anion A of the general formula (1), which can be at least one inorganic anion chosen in particular from F-, Cl-, Bf, I-, (C104)-, (NO3)-, (Cl03)-, (I03)-, (OH)-, (CO3) 2-, (S04) 2-, (S203) 2, (Cr04) 2-, (P04) Z and (B03) 2-, preferably from C1-, (NOs)-, (CO3) 2 and (S04) 2-.

The anion A can also be at least one organic anion chosen from carboxylate anions, preferably mono-, di-or polycarboxylate anions, such as the acetate, oxalate or salicylate anion, and in particular from carboxylate anions with a long aliphatic chain,

in particular a C6 to C12 aliphatic chain, such as the citrate, adipate, caprylate or laurate anion. The anion A can in particular be chosen from anions capable of decomposing thermally, in particular during the heat treatment of the solid precursor (SP). The thermal decomposition can thus be carried out so as to release volatile compounds which, at the time of their escape from the solid, can create a solid with a cellular structure having a high specific surface and comprising in particular numerous pits. The thermal decomposition can be carried out, for example, at a temperature ranging from 200 to 1100°C, preferably from 300 to 950°C, in particular from 300 to 600°C, especially from 300 to 500°C. More particularly, the anion A can be chosen from Cl-, (C03) 2- and (N03)-and from carboxylate anions, which can decompose thermally within the abovementioned temperature ranges with the release in particular of volatile compounds, such as carbon dioxide, nitrogen monoxide, nitrogen dioxide, hydrochloric acid and water vapour.

The solid precursor (SP) can be a natural or synthetic layered double hydroxide and can in particular correspond to the general formula : M2+6N3+2 (OH)-, 6 (Co3) 2-4H2O (2) in which M and N have the same definitions as above, with M2+ representing at least one divalent metal cation of the metal (s) M, in particular chosen from Mg, Zn and Ni, and with N3+ representing at least one trivalent metal cation of the metal (s) N, in particular chosen from Al, Fe, Cr and Mn. It is possible, for example, to choose, as layered double hydroxide, hydrotalcite, manasseite, pyroaurite, sjögrenite, stichtite, barbertonite, takovite, reevesite, desautelsite or coalingite.

The solid precursor (SP) can be a layered double hydroxide having a BET specific surface (determined by the DIN ISO 9277 method) of less than 100 m2/g, in particular ranging from 10 to 90 m2/g. It can be provided in or can be shaped into various forms, in particular in or into the form of pulverulent particles, granules, pellets, tablets, beads or rings, having a mean size which can range from 10 Hm to 50 mm.

The solid precursor (SP) based on LDH can be prepared by synthesis according to one of the methods described, for example, by N. Das et al. in"Catalysis Letters", Vol. 71, No. 3-4 (2001), pages 181 to 185, or by D. Tichit et al. in"Catalysis Today", Vol. 75 (202), pages 197 to 202, or by F. Cavani, F. Trifiro and A. Vaccari in"Catalysis Today", Vol. 11 (1991), pages 201 to 212.

Generally, several processes for the preparation of the solid precursor (SP) based

on LDH are distinguished, in particular processes by coprecipitation, by hydrothermal treatment and by anion exchange. It is particularly recommended to choose the cations and the anions in desired ratios, in particular which are found in the solid precursor (SP), for example in the following molar ratios: 0. 2 < N3+/(M2+ + N+) 0. 4 (3) and 1/n < A°-/N3+ < 1 (4) in which ratios M, N, M2+, N3+, A, An-and n have the same definitions as those given in the general formula (1). The compensating anion, A, generally used is the nitrate or the carbonate.

The most commonly used process is a coprecipitation from solutions (aqueous) of metal salts of the metals present in the LDHs. The coprecipitation can be carried out in various ways, either by varying the pH or, at constant pH, by coprecipitating at slight or at high supersaturation. Generally, a coprecipitation of at least one divalent metal cation and of at least one trivalent metal cation is carried out under conditions in particular of supersaturation which can be achieved either by the physical route, in particular by evaporation, or by the chemical route, in particular by varying the pH. The method by varying the pH, in particular by increasing the pH, for example carried out using sodium hydroxide, potassium hydroxide or sodium bicarbonate (NaHCO3), is often preferred. It is generally recommended to carry out the coprecipitation at a pH equal to or greater than that at which the most soluble hydroxide precipitates. Thus, at a pH ranging from 7.5 to 10, the majority of the metal hydroxides which form LDHs precipitate. The coprecipitation can be carried out at constant pH and at slight supersaturation. In this case, the pH can be controlled by a slow and simultaneous addition of two dilute solutions (for example of 0.5 to 2 mol/litre) in a precipitation chamber. One of the solutions can contain the divalent and trivalent metal cations, in particular the M2+ and N3+ cations, and the other solution can contain sodium hydroxide, potassium hydroxide or sodium bicarbonate. The coprecipitation can also be carried out at constant pH and at high supersaturation. In this case, the solutions containing the metal cations, such as those described above, can be added very rapidly to a solution containing, for example, sodium hydroxide, potassium hydroxide or sodium bicarbonate.

Another process for the preparation of the solid precursor (SP) based on LDH can be a hydrothermal treatment generally comprising the treatment of freshly precipitated mixed metal hydroxides or of mechanical mixtures of metal oxides with water,

optionally in the presence of other anions, so as to synthesize the LDHs and to convert amorphous precipitates to crystalline solids of the type of the LDHs. This can be carried out in an autoclave, at a temperature of greater than 100°C, or else according to an "aging"process at a temperature of less than 100°C.

Another process for the preparation of the solid precursor (SP) based on LDH can be an anion exchange process comprising the treatment of an LDH, having in particular a carbonate anion as"compensating anion", with a dilute solution of acid, such as HCI, HN03 or H2S04, at ambient temperature and for a period of time of a few minutes to a few days.

These three processes can also be used in the preparation of a solid precursor (SP) doped by a metal compound of the metal Me (preparation in which a metal compound comprising the metal Me of the catalytic component is inserted into the solid precursor (SP) before the heat treatment of the latter). Use may in particular be made of the process by coprecipitation as described above, in particular by employing aqueous solutions of metal salts of the metals of the LDHs, in particular the salts of the metals M and N of the general formula (1), and metal salts of the metal Me of the catalytic component. It is thus possible to carry out a coprecipitation of at least one divalent metal cation of M2+ type, of at least one trivalent metal cation of N3+ type and of at least one di-or trivalent metal cation of the metal Me (Me2+ or Me3+). It is also possible to use the process by anion exchange as mentioned above and in which the"compensating anion"of an LDH can be exchanged with a metal anion of the metal Me, such as a polychloride or polyhydroxide anion of the metal Me, for example the [PdCl4] 2-or [Pt (OH) 6] 2-anion.

The heat treatment of the solid precursor (SP), which may or may not be doped by a metal compound of the metal Me, is preferably carried out by thermal decomposition of the precursor, in particular by thermal decomposition of the hydroxide anion and of the"compensating anion"of the LDH. It can be carried out under an inert, oxidizing or reducing atmosphere or alternatively under vacuum. More particularly, it can be carried out under an atmosphere of an inert gas, such as nitrogen or argon, or under an oxidizing atmosphere, such as oxygen, alone or as a mixture with one or more inert gases, for example air, or alternatively under vacuum. The heat treatment can in particular be a calcination, carried out in particular under an atmosphere of air or of air depleted or, in contrast, of air enriched in oxygen. The heat treatment can

advantageously be carried out at a temperature equal to or greater than the temperature at which the solid precursor (SP) begins to decompose and at a temperature in particular below the sintering temperature of the precursor. It is thus possible to choose a temperature ranging from 200 to 1100°C, preferably from 300 to 950°C, in particular from 300 to 600°C, especially from 300 to 500°C.

The heat treatment of a solid precursor (SP) doped by a metal compound of the metal Me can advantageously be carried out in the presence of a reducing agent capable of reducing in particular the metal Me, especially up to an oxidation state of zero. The heat treatment can preferably be carried out under a reducing atmosphere, such as hydrogen, alone or as a mixture with an inert gas.

The heat treatment is carried out under conditions such that the solid precursor (SP) is converted to a support component, in particular based on metal oxide, especially on mixed metal oxides or on a solid solution of metal oxides. The support component resulting from this heat treatment can in particular correspond to the general formula : M (X x) Nxo (5) in which M, N and x have the same definitions as those given in the general formula (1) or (2).

When the solid precursor (SP) is doped by a metal compound of the metal Me, the support component resulting from the heat treatment then comprises the metal Me generally in the form of an oxide. However, when the heat treatment is carried out under a reducing atmosphere, the catalyst can be obtained directly in an active form which is ready directly for the treatment of the alkanolamine.

The heat treatment can also advantageously be carried out under conditions such that the specific surface of the solid precursor (SP) substantially increases, preferably by a factor at least equal to 1.5 and in particular at least equal to 2 or even 3. Thus, the BET specific surface of the support component thus obtained after heat treatment can range from 20 to 300 m2/g, preferably from 100 to 300 m2/g, in particular from 150 to 250 m/g (specific surface measured by the DIN ISO 9277 method). The support component can also be characterized by a mesoporous structure in which the pores can have a diameter ranging from 2 to 50 nm. The support component can be provided in or can be shaped into various solid forms with various mean sizes which are similar or substantially identical to those of the solid precursor (SP), that is to say in or into the form of pulverulent particles, granules, pellets, tablets, beads or rings with a mean size

which can range from 10 llm to 50 mm.

By way of illustration, a preparation of a support component obtained by heat treatment is described in the abovementioned papers by N. Das et al. and by D. Tichit et al.

The catalyst used in the process of the invention comprises an active catalytic component comprising at least one metal Me chosen from Ni, Cu, Ru, Rh, Pd, Ag, Re, Os, Ir and Pt, preferably from Pd, Pt, Ir, Rh, Ag and Cu and in particular from Pd, Pt and Ir. The metal Me present in the catalyst can in particular be found in the metallic state, that is to say in the zero oxidation state. The content by weight of metal Me in the catalyst (calculated as metal Me in the zero oxidation state) can range from 0.05 to 30%, preferably from 0.1 to 20%, in particular from 0.5 to 10%. The catalyst is generally provided in a solid form and with a size of particles which are similar or substantially identical to those of the support component which are described above. It can exhibit a BET specific surface and a pore volume such as those mentioned above.

The catalyst can be prepared according to several processes which depend essentially on the way of inserting the catalytic component into the support component and which depend in particular on the insertion of a metal compound of the metal Me into the solid precursor (SP), the insertion being carried out before or after the heat treatment of the latter.

The process for the preparation of the catalyst can thus comprise the following stages: (a) a stage of heat treatment of the solid precursor (SP) based on LDH, so as to form the support component, (b) a stage of impregnation of the support component using a catalytic precursor (CP) comprising a metal compound or complex of the metal Me, impregnation preferably carried out by a wet route or by a dry route, and (c) preferably a stage of reduction of the catalytic precursor (CP) thus impregnated using a reducing agent capable of reducing the metal Me, in particular up to an oxidation state of zero, in particular hydrogen, the reduction being carried out before or during the process for the treatment of the alkanolamine.

The impregnation stage can be carried out by a wet route (that is to say, in the presence of a liquid) or by a dry route (that is to say, in the absence of a liquid). The impregnation by the wet (or liquid) route can employ a catalytic precursor (CP), in

particular an organic or inorganic metal compound or complex of the metal Me, such as a nitrate, a carbonate, a sulphate, a chloride or an acetylacetonate of the metal Me, pre- ferably in the form of a solution in a liquid medium, such as water or an organic solvent, in particular an aromatic solvent. More particularly, the impregnation by the wet route can comprise an operation in which the support component is brought into contact with the solution of the catalytic precursor (CP), preferably with stirring. After the impregnation stage, the process can comprise a stage of drying and/or of intermediate heat treatment (for example, a decomposition or calcination treatment) of the catalytic precursor (CP) impregnated on the support component, so as in particular to remove the liquid medium and in particular to decompose the catalytic precursor (CP) to a compound of the metal Me, such as an oxide, capable of being reduced, in particular up to an oxidation state of zero. The conditions of the intermediate heat treatment can be identical to those described for the heat treatment of the solid precursor (SP), for example under an inert or oxidizing atmosphere, such as oxygen, or under vacuum, or alternatively under a reducing atmosphere, such as hydrogen. If the intermediate stage takes place under a reducing atmosphere, such as hydrogen, it can be carried out at least in part during the reduction stage.

The impregnation by the wet route can also be carried out by spraying or spray drying. It can also employ the catalytic precursor (CP) described above in the form of a solution of the metal compound or complex of the metal Me in a liquid medium. The impregnation by the wet route can comprise an operation in which the solid support component is suspended in the solution of the catalytic precursor (CP) and an operation in which the suspension is sprayed using a sprayer or spray dryer. As above, a stage of intermediate heat treatment of the catalytic precursor (CP) impregnated on the support component can be carried out, that is to say under an inert, oxidizing or reducing atmosphere and preferably under a pressure of less than atmospheric pressure.

The impregnation can also be carried out by the dry route, in particular by sublimation of the catalytic precursor (CP) onto the support component. The sub- limation can be carried out under an inert, oxidizing or reducing atmosphere and preferably under a pressure of less than atmospheric pressure. In particular, the sublimation can be carried out under temperature and pressure conditions which make it possible to provide the sublimation, the migration and the deposition of the catalytic precursor (CP) on the support component. The sublimation can take place in the

presence of a reducing agent capable of reducing the metal Me, in particular up to an oxidation state of zero, in particular under a reducing atmosphere, such as hydrogen. In this case, it can be carried out at least in part during the reduction stage.

The reduction stage comprises a reduction of the catalytic precursor (CP) impregnated on the support component, so as to reduce the oxidation state of the metal Me, in particular to zero. The reducing agent can be hydrogen, alone or as a mixture of one or more inert gases, such as nitrogen or argon. The reduction can be carried out by bringing the support component, impregnated with the catalytic precursor (CP), into contact with the reducing agent, in particular with hydrogen. It can be carried out at a temperature ranging from 20 to 400°C, at atmospheric pressure or under a pressure greater than atmospheric pressure. When the reduction takes place at atmospheric pressure, the reduction stage can comprise an increase in the temperature under an inert atmosphere, such as nitrogen, argon, methane, ethane or any other alkane, up to the reduction temperature, such as that mentioned above, and subsequently the gradual replacement of the inert atmosphere by hydrogen. When the reduction takes place under a pressure greater than atmospheric pressure, the reduction stage can be carried out under temperature and pressure conditions similar to those used subsequently in the treatment of the alkanolamine. The duration of the reduction stage generally depends on the temperature and on the pressure, in particular on the hydrogen pressure.

The process for the preparation of the catalyst can also comprise the following stages : (a) a stage of insertion of a catalytic precursor (CP) comprising a compound, a complex or a metal anion of the metal Me into the solid precursor (SP) based on LDH, insertion preferably carried out by coprecipitation or by anion exchange, so as to form a solid precursor (SP) doped by a metal compound of the metal Me, (b) a stage of heat treatment of the solid precursor (SP) thus doped, so as to form a support component of the catalyst comprising a metal compound of the metal Me, and (c) preferably a stage of reduction of the metal compound of the metal Me present in the support component using a reducing agent capable of reducing the metal Me, in particular up to an oxidation state of zero, especially of hydrogen, the reduction being carried out before or during the process for the treatment of the alkanolamine.

The catalytic precursor (CP) can be that described above in the preparation of a solid precursor (SP) doped by the metal compound of the metal Me. The heat treatment and reduction stages can be identical to those described above.

The catalyst which comprises the metal Me in a nonreduced form, in particular at an oxidation state of greater than zero, can be used directly in the treatment of the alkanolamine according to the invention. In this case, the reduction of the metal Me dispersed or distributed in the support component can take place simultaneously with the treatment of the alkanolamine.

The alkanolamine used in the process according to the invention can be prepared according to known processes, for example by reaction of ammonia or of a primary or secondary amine with ethylene oxide or propylene oxide. The alkanolamine which is preferably used is an ethanolamine or a propanolamine and can be chosen in particular from monoethanolamine (MEA), diethanolamine (DEA), triethanolamine (TEA), ethoxylated triethanolamines (ETEA) (also called triethanolamine glycol ethers), aminoethylethanolamine (AEEA) (also called hydroxyethylethylenediamine, corresponding to the general formula: NH2-CH2-CH2-NH-CH2-CH20H), monoiso- propanolamine, diisopropanolamine, triisopropanolamine, ethoxylated triisopropanol- amines (also called triisopropanolamine glycol ethers) and mixtures of two or more of these, and more particularly from TEA, AEEA, ETEAs and mixtures of TEA with MEA, DEA and/or ETEAs.

By way of illustration, the ethanolamines and in particular TEA can be prepared by reaction of ammonia with ethylene oxide according to one of the processes described in"SRI International, Process Economics Program Report", No. 193 (January 1993), pages 6-1 to 6-9. Thus, TEA can be prepared by bringing ethylene oxide into contact with ammonia, for example in a molar ratio of the ammonia to the ethylene oxide ranging from 0. 5/1 to 40/1, in particular from 1/1 to 10/1. The reaction can take place in an aqueous medium, so that the ratio by weight of the ammonia to the water can be from 0. 1/1 to 1/1. The reaction can be carried out at a temperature ranging from 0 to 150°C, in particular from 20 to 100°C, especially from 40 to 80°C, and under an absolute pressure which can range from 0.1 to 15 MPa, in particular from 0.2 to 5 MPa, especially from 0.2 to 2 MPa. It is thus possible to manufacture, in an aqueous medium, a"crude"TEA comprising TEA as a mixture with one or more other ethanolamines produced simultaneously during the reaction, such as MEA, DEA and ETEAs, and

generally an excess of at least one of the two unreacted starting reactants, in particular an excess of ammonia. The preparation of the TEA can subsequently comprise one or more stages of purification, in particular by fractionation or distillation, intended to separate and to recover the TEA from the"crude"TEA. Thus, one of these purification stages can comprise a separation of the"crude"TEA from the aqueous medium in which the TEA was manufactured, and from the excess of one of the two starting reactants, in particular ammonia. Such a separation is generally carried out by two successive distillations. Another stage of purification of the TEA can make it possible to separate and to isolate the"purified"TEA from the"crude"TEA. It can, for example, comprise a separation, in particular by distillation, of the TEA from the other ethanolamines, as mentioned above, produced simultaneously with the TEA.

The process of the invention is particularly well suited to one of the alkanolamines mentioned above, in particular a TEA manufactured according to the one of the processes described above, in particular to one of the"crude"or preferably "purified"alkanolamines, in particular a"crude"or"purified"TEA, as described above.

"Purified"alkanolamine (or TEA) is generally understood to mean an alkanolamine (or TEA) having a content by weight of alkanolamine (or TEA) equal to or greater than 85%, preferably equal to or greater than 97%, in particular equal to or greater than 99%.

Generally, the alkanolamines (or in particular TEA) thus prepared and used according to the invention can have a colour index of less than or equal to 100 Pt/Co, preferably of less than or equal to 50 Pt/Co, in particular of less than or equal to 40 or 30 Pt/Co, measured according to the ASTM D 1209 method. However, these ethanolamines (or in particular TEA) can generally undergo a deterioration in their colour over time, in particular during their storage, so that their colour index can relatively quickly reach values much greater than those mentioned above.

The process of the invention specifically comprises a process for the treatment of an alkanolamine intended to improve and also to stabilize the colour of the alkanolamine over time, in particular at high temperatures. The treatment comprises an operation in which an alkanolamine is brought into contact with hydrogen in the presence of the catalyst as described above. It can be carried out at a high temperature, for example a temperature ranging from 30 to 200°C, preferably from 50 to 180°C, in particular from 60 to 160°C, especially from 70 to 150°C.

The treatment of the alkanolamine with hydrogen can be carried out at

atmospheric pressure or under a pressure greater than atmospheric pressure, for example under a hydrogen partial pressure ranging from 0.1 to 5 MPa, preferably from 0. 1 to 3 MPa, in particular from 0. 1 to 2 MPa. In this treatment, the hydrogen can generally be used in a large molar excess with respect to the alkanolamine to be treated. The hydrogen can be used alone or as a mixture with one or more inert gases, such as nitrogen, argon, methane, ethane or any other alkane. The catalyst can be present in an extremely variable amount with respect to the alkanolamine to be treated, which amount can depend in particular on the temperature of the treatment, on the mean residence time of the alkanolamine with the catalyst and on the initial colour index of the alkanolamine.

The treatment of the alkanolamine with hydrogen in the presence of the catalyst can be carried out batchwise, for example in a stirred reactor. It can also be carried out continuously, for example in one or more tubular reactors : the catalyst can in particular be positioned inside the tubes of the reactor, for example in the form of a stationary bed, or alternatively can be entrained in the form of a suspension with the alkanolamine while the alkanolamine moves under a hydrogen atmosphere continuously through the tubes, in particular in the form of an upward stream, a downward stream or in a loop (that is to say, by a movement comprising several successive passes or passages through the tubes). The process can also be carried out continuously in one or more stirred reactors, in particular arranged in series or in cascades. Thus, the catalyst can be positioned in the reactor (s), for example in the form of a stationary bed, while the alkanolamine moves through this (these) reactor (s) in the form of a continuous stream in a hydrogen atmosphere. It is also possible for a suspension of the catalyst in the alkanolamine to move through the reactor (s) in the form of a continuous stream under a hydrogen atmosphere, the stream preferably moving in a loop through this (these) reactor (s). Thus, generally, in the process of the invention, the catalyst can be employed either in the form of a stationary bed or in the form of a bed entrained or of a suspension in the alkanolamine to be treated.

The mean residence time of the alkanolamine with the catalyst generally results from the initial colour index of the alkanolamine (that is to say, before treatment) and from the extent of the decolouration desired, as well as from the degree of stability or of instability of the alkanolamine to be treated. It can also depend on the fact that the catalyst comprises the metal Me in a condition which may or may not be already

reduced, that is to say at an oxidation state of zero or, in contrast, of greater than zero.

As a general rule, the greater the extent of the desired decolouration, the longer the mean residence time of the alkanolamine with the catalyst. Likewise, the greater the colour index of the initial alkanolamine, the longer the mean residence time. If the metal Me in the catalyst used is in a nonreduced condition, that is to say if the catalyst comprises the metal Me at an oxidation state of greater than zero, the mean residence time will be longer than if the oxidation state of the metal Me were equal to zero.

Thus, depending on all these conditions, the mean residence time can be highly variable and can range generally from a few seconds to a few hours, preferably from 10 seconds to 20 hours, in particular from 1 minute to 12 hours.

In a continuous process carried out with a stationary bed of catalyst, the hourly space velocity for passage of the alkanolamine over the catalyst can be, for example, from 0.2 to 200 kg of alkanolamine per litre of catalyst and per hour, it being known that the volume of catalyst corresponds to the bulk volume of the catalyst. Preferably, the hourly space velocity can be from 0.5 to 150 kg/l. h (according to the preceding units).

At the end of treatment, it is possible to separate and recover, on the one hand, the catalyst and, on the other hand, the treated alkanolamine. This can be carried out by separation by settling, filtration and/or centrifuging, in particular if it is desired to recover and reuse the catalyst in subsequent and similar treatments.

It has been found, according to the invention, that the alkanolamine thus treated in the presence of this catalyst exhibits a colour which is not only improved but also a colour stability over time which is substantially improved, in particular at high temperatures, for example of greater than ambient temperature (20°C), and in particular temperatures ranging from 30 to 140°C, preferably from 40 to 140°C, and especially under an atmosphere of air. The alkanolamine thus treated according to the invention can have a colour index ranging from 0 to 40 Pt/Co, preferably from 0 to 30 Pt/Co (measured according to the ASTM D 1209 method).

An aging test under hot conditions and under air on an alkanolamine makes it possible to determine the level of thermal stability of the alkanolamine. This test, carried out in particular under air at 140°C, makes it possible to intensify the phenomena of thermal instability of the alkanolamine with respect to the ambient and natural conditions of recolouration and also to extenuate these phenomena with respect

to an aging test under hot conditions carried out under an inert atmosphere, such as nitrogen. It is thus capable of revealing appreciable differences between various treat- ments applied to the alkanolamines. More particularly, the aging test under hot con- ditions and under air consists in introducing 50 g of an alkanolamine, in particular a TEA, into a glass container with stirring and under an atmosphere of air at atmospheric pressure and in then heating the container at 140°C for 4 hours. By the end of this time, the container is cooled to ambient temperature (20°C), the colour index (Iz) of the alkanolamine is then measured (according to the ASTM D 1209 method) and the index 12 is compared with the colour index (Il) of the alkanolamine measured according to the same method before the aging test. The difference in colour index (AI = I2-It) makes it possible to reveal the level of thermal stability of the alkanolamine with respect to the phenomena of recolouration over time. The smaller the difference in colour index (AI), the better the thermal stability of the alkanolamine with respect to the phenomena of recolouration.

The present invention also relates to the use of a catalyst in a process for the treatment of an alkanolamine for improving and stabilizing the colour of the alkanolamine over time, in particular at high temperatures, which process comprises bringing the alkanolamine into contact with hydrogen in the presence of a catalyst, which use is characterized in that the catalyst comprises an active catalytic component comprising at least one metal Me chosen from Ni, Cu, Ru, Rh, Pd, Ag, Re, Os, Ir and Pt and a support component formed by heat treatment of a solid precursor (SP) based on layered double hydroxide (LDH) comprising at least one divalent metal cation and at least one trivalent metal cation.

All the preferred characteristics described above relating to the solid precursor (SP), the heat treatment of the precursor (SP), the support component obtained in particular after heat treatment, the active catalytic component and in particular the metal Me, the catalyst and the preparations for obtaining it, as well as the conditions of the treatment of the alkanolamine, are also valid in the use of this catalyst according to the invention.

The following examples illustrate the present invention.

Example 1 : Preparation of a catalyst based on Pd supported on a layered double hydroxide (LDH) which is heat treated (a) Preparation of a layered double hydroxide (LDH)

A layered double hydroxide (LDH), in particular of hydrotalcite type, was synthesized by coprecipitation of a gel under air at constant pH. 250 cm3 of a first aqueous solution (A) of Mg (N03) 2-6H20 and Al (N03) 3-9H20 (in a molar ratio Mg/Al = 2) were introduced into a reactor using a pump according to a flow rate of 1 cm3/min and, simultaneously, a second aqueous 2M NaOH + 0. 5M Na2C03 solution was introduced using a pH stat device sold by Metrohm under the reference"718 Stat Titrino (g)". The control of the flow rate made it possible to maintain the pH during the precipitation at a constant value equal to 10.0 i 0.2. At the end of the precipitation, a suspended gel was obtained which was aged at 80 : h 5'C for 15 h with stirring. The solid thus obtained was subsequently isolated by centrifuging and washed with distilled water.

Exchange of the nitrate anions by carbonate anions was subsequently carried out by dispersing 2 g of the preceding solid in a 1.5 x 10-3M aqueous Na2C03 solution with stirring at 80°C for 2 h. After filtration, a solid precursor (SP) based on layered double hydroxide (LDH), in particular of hydrotalcite type, was recovered, which precursor was washed with distilled water and dried in an oven at 80°C and corresponded to the general formula: [Mg0.7Al0.33(OH)2]0.33+(CO32-)0.165#0.5H2O (6) (b) Preparation of a support component 2 g of the solid precursor (SP) were introduced into an oven at ambient temperature (20°C) and were heat treated by calcination in a stream of dry air at a flow rate of 100 cm3/min, the temperature being increased from 20 to 450°C at the rate of 3°C/min and being maintained at 450°C for 3 h. After cooling to ambient temperature (20°C), a support component based on mixed oxides of Mg and of Al was thus obtained.

(c) Preparation of a supported catalyst based on Pd 100 g of the support component thus obtained were introduced into a reactor at ambient temperature and was brought into contact with 10.8 g of Pd (N03) 2 in an aqueous solution (sold by Strem Chemicals) for 12 h. After evaporation, the solid was dried at 80°C under vacuum and was then calcined under a stream of air at 350°C for 4 h. The solid thus calcined was subjected to a reduction reaction using a gaseous mixture of hydrogen and nitrogen (in a ratio by volume respectively of 10/90) moving with a flow rate of 60 cm3/min/g of solid at 200°C for 5 h. After cooling to ambient temperature, a catalyst based on Pd metal on a support of layered double hydroxide which was heat treated comprising 5% by weight of Pd metal, was thus obtained.

Example 2: Treatment of TEA with the catalyst of Example 1 70 g of a TEA having a purity of 99.4% and an initial colour index of 63 Pt/Co (determined by the ASTM D 1209 method) and 0.5 g of the catalyst prepared in Example 1 were introduced with stirring into a reactor. The reactor was heated to 120°C and hydrogen was introduced therein so as to have a hydrogen partial pressure of 2 MPa. After treatment for one hour, the colour index of the TEA thus treated had fallen to a value (Il) of 20 Pt/Co (ASTM D 1029).

A sample of the TEA thus treated was subjected to the aging test under hot conditions and under air at 140°C for 4 h as described above. At the end of this time, a TEA was obtained with a colour index which had climbed to a value (12) of 75 Pt/Co (ASTM D 1209), so that the difference in colour index (AI = I2-Il) was equal to 55.

By way of illustration, the initial untreated TEA was also subjected to the same test of aging under hot conditions and under air. At the end of this test, a highly coloured TEA was obtained with a colour index which had changed from 63 Pt/Co (before the test) to more than 500 Pt/Co (after the test).

Example 3 (comparative) : Treatment of TEA with a catalyst based on Pd supported on active charcoal The treatment was carried out exactly as in Example 2 except that the catalyst of Example 1 was replaced by a catalyst based on Pd supported on an active charcoal comprising 5% by weight of Pd metal, manufactured by Johnson Mattey and sold by Alpha. After treatment for one hour, the colour index of the TEA thus treated had fallen to a value (Il) of 20 Pt/Co (ASTM D 1209 method).

A sample of the TEA thus treated was subjected to the same test of aging under hot conditions and under air as that of Example 2. The TEA was thus obtained with a colour index which had risen to a value (I2) of 130 Pt/Co (ASTM D 1209), so that the difference in colour index (AI) was equal to 110, i. e. to a value significantly greater than that of Example 2.

Example 4 (comparative) : Treatment of TEA with a Raney Ni catalyst The treatment was carried out exactly as in Example 2 except that the catalyst of Example 1 was replaced by a Raney Ni catalyst sold by Strem Chemicals. After treatment for one hour, the colour index of the TEA thus treated had fallen to a value (I,) of 40 Pt/Co (ASTM D 1209).

A sample of the TEA thus treated was subjected to the same test of aging under

hot conditions and under air as that of Example 2. A TEA was thus obtained with a colour index which had risen to a value (I2) of 200 Pt/Co (ASTM D 1209), so that the difference in colour index (AI) was equal to 160, i. e. to a value very significantly greater than that of Example 2.

Example 5: Preparation of a catalyst based on Pd on a layered double hydroxide (LDH) which is heat treated (a) Preparation of a layered double hydroxide (LDH) doped with Pd A layered double hydroxide (LDH) doped with Pd was synthesized by coprecipitation under air at constant pH. 250 cm3 of a first aqueous solution (B) of Mg (NO3) 2 6H20, Pd (N03) 2 and al (N03) 3-9H20 (in molar ratios Mg/Al = 2 and Pd/Al = 0.06) were introduced into a reactor using a pump at a flow rate of 1 cm3/min and, simultaneously, a second aqueous solution of 2M NaOH + 0. 5M Na2CO3 was introduced using a pH stat device sold by Metrohm under the reference"718 Stat Titrino" (g). The control of the flow rate made it possible to maintain the pH during the coprecipitation at a constant value equal to 10.0 i 0.2. At the end of the coprecipitation, a suspended solid was obtained which was aged at 80 d : 5°C for 15 h with stirring. The solid thus obtained was subsequently isolated by centrifuging and washed with distilled water.

Exchange of the nitrate anions by carbonate anions was subsequently carried out by dispersing 2 g of the preceding solid in a 1.5 x 10-3M aqueous Na2C03 solution with stirring at 80°C for 2 h. After filtration, a solid precursor (SP) based on layered double hydroxide doped by a Pd compound was recovered, which precursor was washed with distilled water and dried in an oven at 80°C and corresponded to the general formula: [Pdo o2Mgo 67Alo. 31(OH)2]0.31+(CO32-)0.155#0. 5H20 (7) (b) Preparation of a support component doped with Pd 2 g of the solid precursor (SP) thus doped were introduced into an oven and were heat treated by calcination in a stream of air at a flow rate of 100 cm3/min, the temperature being increased from 20 to 450°C at the rate of 3°C/min and being maintained at 450°C for 3 h. After cooling to ambient temperature (20°C), a support component doped by a Pd compound was thus obtained, which component was based on mixed oxides of Mg, of A1 and of Pd.

(c) Preparation of a supported catalyst based on Pd The support component prepared above was subjected to a reduction reaction. The support component was introduced into a reactor and then the reactor was heated to

200°C. A gaseous mixture of hydrogen and nitrogen (in a ratio by volume respectively of 10/90) was subsequently introduced therein at a flow rate of 60 cm3/min/g of the component for 5 h. At the end of this time, the reactor was cooled at ambient temperature and a catalyst based on Pd metal on a support of layered double hydroxide which was heat treated comprising 5% by weight of Pd metal, was obtained.

Example 6: Preparation of a catalyst based on Ni supported on a layered double hydroxide which is heat treated (a) Preparation of a layered double hydroxide (LDH) doped with Ni The preparation was carried out exactly as in Example 5 (a) except that, instead of using the aqueous solution (B), an aqueous solution ofMg (N03) 2-6H20, Ni (NO3) 2-6H20 and Al (N03) 3-9H20 (in molar ratios Mg/Al = 2.16 and Ni/Al = 0.06) was used. After coprecipitation and exchange of the nitrate anions by carbonate anions, a solid precursor (SP) based on layered double hydroxide doped with a Ni compound was thus obtained which corresponded to the general formula: [Nio. o2Mgo. 67Alo 3i (OH) 2]0.31+(CO32-)0.155#0.5H2O (8) (b) Preparation of a support component doped with Ni The preparation was carried out exactly as in Example 5 (b) except that the solid precursor (SP) doped with a Ni compound prepared above was used. A support component doped by a Ni compound was thus obtained, which component was based on mixed oxides of Mg, of Al and of Ni.

(c) Preparation of a supported catalyst based on Ni The preparation was carried out exactly as in Example 5 (c) except that the support component prepared above was used. A catalyst based on Ni metal on a support of layered double hydroxide which was heat treated comprising 5% by weight of Ni metal, was thus obtained.

Example 7: Preparation of a catalyst based on Pt supported on a layered double hydroxide which is heat treated (a) Preparation of a layered double hydroxide (LDH) The preparation was carried out exactly as in Example l (a).

(b) Preparation of a support component The preparation was carried out exactly as in Example 1 (b).

(c) Preparation of a supported catalyst based on Pt The preparation was carried out exactly as in Example l (c) except that, instead of using

the aqueous Pd (N03) 2 solution, an anhydrous solution in toluene of Pt acetylacetonate, Pt (C5H702) 2, sold by Strem Chemicals, was used. A catalyst based on Pt metal on a support of layered double hydroxide which was heat treated comprising 5% by weight of Pt metal, was thus obtained.

Example 8: Preparation of a catalyst based on Pd supported on a layered double hydroxide which is heat treated (a) Preparation of a layered double hydroxide (LDH) The preparation was carried out exactly as in Example l (a) except that, instead of using the aqueous solution (A), an aqueous solution of Zn (NO3) 2 6H20 and A1 (N03) 3-9H20 (in a molar ratio Zn/Al = 2) was used. After coprecipitation and exchange of the nitrate anions by carbonate anions, a solid precursor (SP) based on layered double hydroxide was thus obtained which corresponded to the general formula: [Zn0.67Al0.33(OH)2]0.33+(CO32-)0.165#0.5H2O (9) (b) Preparation of a support component The preparation was carried out exactly as in Example l (b) except that the solid precursor (SP) prepared above was used. A support component based on mixed oxides of Zn and of Al was thus obtained.

(c) Preparation of a supported catalyst based on Pd The preparation was carried out exactly as in Example l (c) except that the support component prepared above was used. A catalyst based on Pd metal on a support of layered double hydroxide which was heat treated comprising 5% by weight of Pd metal, was thus obtained.

Example 9: Preparation of a catalyst based on Pd and Pt supported on a layered double hydroxide (LDH) which is heat treated (a) Preparation of a layered double hydroxide (LDH) The preparation was carried out exactly as in Example 1 (a).

(b) Preparation of a support component The preparation was carried out exactly as in Example l (b).

(c) Preparation of a supported catalyst based on Pd and Pt The preparation was carried out exactly as in Example l (c) except that, instead of using the aqueous Pd (N03) 2 solution, an anhydrous solution of Pd acetylacetonate, Pd (C5H702 2, and of Pt acetylacetonate, Pt (CsH702) 2, (which are sold by Strem Chemicals) in equimolar amounts in toluene was used. A catalyst based on Pd metal

and on Pt metal on a support of layered double hydroxide (LDH) which was heat treated comprising 3.2% by weight of Pd metal and 1.8% by weight of Pt metal, was thus obtained.

Example 10: Treatments of TEA successively with the catalysts of Examples 5 to 9 The treatments were carried out exactly as in Example 2 except that the catalyst of Example 1 was replaced successively for each of the catalysts of Examples 5 to 9. After treatment for one hour, the colour index of the TEA thus treated successively in the presence of the different catalysts had fallen to low values, as in Example 2.

Samples of the TEA thus treated successively in the presence of the different catalysts were subjected to the same test of aging under hot conditions and under air as that of Example 2. A TEA with a colour index which had risen relatively little, so that the difference in the colour indices measured after and before the aging test remained relatively low, was thus obtained for each of the tests, as in Example 2.