EP1532086A1

EP1532086A1 - Process for the catalytic hydrogenation of a substance

Info

Publication number: EP1532086A1
Application number: EP03752771A
Authority: EP
Inventors: Stephan Andreas Schunk
Original assignee: HTE GmbH; HTE GmbH the High Throughput Experimentation Co
Current assignee: HTE GmbH; HTE GmbH the High Throughput Experimentation Co
Priority date: 2002-05-21
Filing date: 2003-05-20
Publication date: 2005-05-25
Also published as: WO2003097561A1; AU2003232804A1; DE10222385A1; DE10222385B4

Abstract

The present invention relates to a process for hydrogenating at least one organic substrate in at least two liquid phases which are at least partially immiscible with one another using a catalyst, wherein at one of the two or more liquid phases contains at least the following further components: (i) at least one surface-active substance and (ii) at least one salt which is at least partially soluble in at least one of the at least two liquid phases.

Description

Process for the catalytic hydrogenation of a substrate

The present invention relates to a process for hydrogenating at least one organic substrate in at least two liquid phases which are at least partially immiscible with one another using a catalyst, wherein at least one of the two or more liquid phases contains at least the following further components: (i) at least one surface-active substance and (ii) at least one salt which is at least partially soluble in at least one of the liquid phases.

The present invention is in the fields of catalysis and of multiphase reactions. Among catalysts, a distinction is made between homogeneous and heterogeneous catalysts. In contrast to homogeneous catalysis in which all reactants are present in one phase, in the case of heterogeneous catalysis, reactants and products are present in a different phase from the catalyst. The advantage of heterogeneous catalysis over homogeneous catalysis is the greater ease with which the catalyst can be separated off and the resulting lower costs for purifying the product obtained and for regenerating and/or removing the catalyst.

The heterogeneously catalysed hydrogenation of an organic substrate in the liquid phase described in the present invention is a multiphase reaction in which at least the following three phases are present: a gas phase and at least two liquid phases which are at least partially immiscible with one another. Two phases which are at least partially immiscible with one another, e.g. an "oil phase" (hydrophobic, immiscible with water) and a "water phase" (hydrophilic, miscible with water) , are regarded as different phases for the purposes of the invention. If, for example, a solid catalyst, hydrogen as reactive gas and an aqueous phase and an oil phase are present, then this is overall a four-phase reaction. A very large interfacial area between the respective phases is in this case of particular importance, for example for satisfactory mass transfer. It has also been found that hydrogenation reactions are catalysed with increased activity and/or selectivity when oil and water phases are present together (cf. the prior art described below) .

The prior art regarding the hydrogenation of organic compounds in multiphase reactions using a heterogeneous catalyst is described below. The organic compounds in question are usually aromatic.

Thus, for example, EP 0 554 765 Al describes a process for preparing cyclohexene by partial hydrogenation of benzene by means of hydrogen in the presence of water and a ruthenium catalyst which is modified with nickel. The document focuses on an improved catalyst and provides no teachings regarding the addition of further substances which are at least partially soluble in the liquid phase.

On the other hand, US 5 969 202 pays particular attention not only to the catalyst but also to the composition of the liquid phase. In particular, it is assumed that the starting materials and products are preferentially present in the oil phase while the aqueous phase comprises mostly the cosolvent water. Control of this phase separation makes it possible to control the selectivity to individual products, in this case cyclic olefin relative to cyclic alkane, for example by addition of zinc sulphate. However, apart from the zinc sulphate, no further substances for controlling the phase behaviour and the selectivity are disclosed. A further document which describes the addition of salts is, for example, US 5 973 218.

The prior art relating to organic reactions in oil/water systems, i.e. in systems having at least two liquid phases which are immiscible with one another, includes, in particular, phase transfer catalysis. For the purposes of the present invention, this is the acceleration of reactions in two-phase systems (e.g. water/organic solvent) in which the substrate is present in the lipophilic (hydrophobic) organic phase and the reagent is, for example^*, present in the aqueous or solid phase. Transport of the reagent through the phase boundary to the substrate is then effected by phase transfer catalysis. According to the prior art, lipophilic quaternary ammonium salts with a counterion, for example tetrabutylammonium hydrogensulphate or methyltrioctylammonium chloride, are mostly used for this purpose. Phase transfer catalysts belong to the class of surface-active substances as are defined further below in the present invention.

As an example of the use of a phase transfer catalyst for the (partial) hydrogenation of organic substrates, mention may here be made of US 5 599 962 in which an Ru complex, i.e. not a solid and thus not a heterogeneous catalyst, and also gaseous hydrogen, water, an organic solvent and a phase transfer catalyst of the abovementioned type are used.

Overall, in respect of the hydrogenation of organic substrates in multiphase reactions in the presence of heterogeneous catalysts, the prior art teaches the use of a range of different catalysts, in particular catalysts based on Ru. In this context, the use of salts to increase the selectivity is also known. In another context, the use of surface-active substances as phase transfer catalysts in the hydrogenation of organic substrates is known for mixtures where at least two liquid phases which are immiscible with one another are present and homogeneous catalysts are used.

It is an object of the present invention to improve the selectivity or the yield or the selectivity and the yield in the hydrogenation of organic substrates in a multiphase reaction using a heterogeneous catalyst over that/those achieved in the prior art.

The object of the invention is achieved by providing the following process: hydrogenation of at least one organic substrate in at least two liquid phases which are at least partially immiscible with one another using a catalyst, wherein at least one of the two or more liquid phases contains at least the following further components: (i) at least one surface-active substance and (ii) at least one salt which is at least partially soluble in at least one of the liquid phases.

For the purposes of the present invention, the substrate used can in principle be any organic compound which reacts with hydrogen. Preference is given to using compounds which have at least one bond selected from the following group: C-C double bonds, C-C triple bonds, aromatic systems, C-N double bonds, C-N triple bonds, C-0 double bonds and C-S double bonds. These bonds can be present in a monomer or a polymer. For the purposes of the present invention, aromatic hydrocarbons are particularly preferred. The aromatic substrate or substrates can be selected from the group consisting of benzene, benzene substituted on the ring by at least one hydroxy, carboxy or amino group, benzene derivatives having at least one alkyl group, polycyclic aromatics, fused aromatics, heteroaromatics, multinuclear aromatics and also nonbenzenoid aromatics. Benzene and toluene and their derivatives are particularly preferred.

The organic substrate and optionally a solvent in which this substrate is at least partially soluble is referred to as solvent. The solvent in question is at least partially immiscible with the cosolvent described below. The solvent is typically the hydrophobic "oil phase" .

Correspondingly, the liquid which is at least partially immiscible with the organic substrate or is less soluble with the organic substrate, is referred to as cosolvent for the purposes of the present invention. Figure 1 shows, by way of example, the influence of the use of monools and polyols on the reaction rate in the hydrogenation of toluene in the presence of different proportions of cosolvent. The reaction conditions were: hydrogenation pressure 70 bar; temperature 60°C; total volume = 30 ml; volume ratio of toluene/cosolvent variable; Ru/C catalyst (25 mg) . The homologous series methanol, glycol and glycerol was examined here. Water serves as comparative system in this case and also for the examples described below, since the hydrogenation of benzene, for example, is carried out industrially in a water phase.

In principle, any ratio of cosolvent to solvent (i.e., for example, the ratio of water to toluene) is conceivable. However, preference is given to a ratio of from 10:1 to 1:10, with particular preference being given to a value of from 1:1 to 10:1 and further preference being given to a value of form 5:1 to 10:1.

For the purposes of the present invention, the hydrogenation can in principle be carried out using any hydrogen-donating substance. The hydrogen can be introduced into the reaction system from the outside as gas, preferably as hydrogen gas or as a gas mixture containing hydrogen. However, the hydrogen can also be generated in the liquid phase by reaction or from precursor compounds, for example as nascent hydrogen.

The hydrogenation reaction can be any reaction in which an organic substrate reacts with at least one of the above-described hydrogenating agents. For the purposes of the present invention, selective hydrogenations are particularly preferred.

For the purposes of the present invention, a selective hydrogenation is any hydrogenation in which at least one target product is formed in higher yield than any other single product, including by-products and/or waste products. Complete hydrogenation is explicitly included as a possibility, but a partial hydrogenation is preferred. A partial hydrogenation is, for the purposes of the present invention, any hydrogenation which does not lead to complete hydrogenation of the substrate.

The catalyst to be used for the hydrogenation is, for the purposes of the present invention, not subject to any restrictions at all. The catalyst can in principle also be homogeneous, i.e. be present as a solution in one of the liquid phases. However, for the purposes of the present invention, preference is given to heterogeneous, i.e., solid, catalysts. An advantage of the present invention is, in particular, that catalysts which do not come into question for industrial processes, e.g. hydrogenation processes, due to nonoptimal activity and/or selectivity can also be used industrially as a result of the additives employed according to the invention.

In a preferred embodiment, the catalysts used according to the invention comprise at least one metal, preferably a catalytically active metal ("active metal") selected from the group consisting of metals of the main groups and transition groups of the Periodic Table of the Elements. The metals Cu, Ni and Co and the metals of the platinum group, i.e. Ru, Rh, Pd, Re, Os and Ir, are particularly preferred.

In a further preferred embodiment, the metal or metals is or are applied to a porous support or is or are a constituent of a porous material. As porous (support) material, in principle any material which is porous and withstands the conditions occurring during use of the catalyst can be used. Refractory, i.e. nondecomposing, oxides and also related mixed oxides and/or oxide mixtures are of particular importance. For the purposes of the invention, preference is given to using those porous support materials which have been found to be generally useful in the catalysis of hydrogenation.

Particular preference is given to silicates, in particular aluminosilicates and more preferably zeolites and also titanium oxides, aluminium oxides, silicon oxides, zirconium oxides, carbon-containing materials, carbon modifications and especially activated carbon, or mixtures of at least two of the abovementioned substances. Grids, meshes, braided materials, honeycombs or compositions of metals or of composite materials containing metals and/or ceramics are likewise conceivable. The use of a nonporous support and/or application of the catalytically active material, for example at least one active metal, as a film or layer to a support is likewise conceivable.

The catalytically active transition metals are preferably used in the form of their salts and particularly preferably in the form of their oxides or sulphides as supported catalysts or as pure metal in the form of all-active or supported catalysts. These catalysts can be used as powder catalysts which are present in the reactor as a fine suspension or as pellets, tablets or granules. in a preferred embodiment, the catalysts to be used should be at least not completely soluble in the liquid phase. Greater preference is given to the heterogeneous catalyst not being soluble at all, i.e. it has a low solubility product as is typical for a transition metal oxide, both in the solvent and in the cosolvent.

For the purposes of the present invention, surface- active substances are all substance which, in liquid systems, accumulate or are depleted at at least one phase boundary selected from the group consisting of liquid/gas, liquid/liquid and liquid/solid phase boundaries. The terms "surfactant" and "surface-active substance" are used synonymously for the purposes of the present invention. A characteristic embodiment of surfactants comprises bipolar compounds in which the hydrophilic part comprises one or more polar groups. The hydrophobic part preferably comprises a chain of carbon atoms .

For the purposes of the present invention, preference is given to anionic, cationic, nonionic and ampholytic surfactants. Anionic surfactants are particularly preferred for the purposes of the present invention. Anionic surfactants may have carboxylate, sulphate or sulphonate groups. The use of cationic surfactants having at least one quaternary ammonium group or nonionic surfactants is also conceivable. This also applies to ampholytic surfactants, i.e. surface-active substances which contain both anionic and cationic groups and accordingly behave as anionic or cationic surface-active compounds depending on the pH. Furthermore, surface-active substances selected from the group consisting of block copolymers, poly (alkylene oxide) triblock copolymers; alkylpoly (ethylene oxides); lipids, phospholipids; and also combinations of two or more of the abovementioned substances are also preferred. Among nonionic surface-active substances, preference is given to compounds containing at least one ether group. Polyethers such as polyethylene glycol or polypropylene glycol are particularly preferred.

The use of polyethylene-block-poly (ethylene glycol), poly (ethylene glycol) 4-nonylphenyl 3-sulphopropyl diether, poly (ethylene glycol) monolaurate, hexadecyl- trimethylammonium acetate, hexadecyltrimethylammonium nitrate and trimethylammonium bromides, for example hexadecyltrimethylammonium bromide (CTAB) , is particularly preferred.

The proportion of surface-active substance to be used is in principle not subject to any restrictions. In principle, preference is given to the value at which the optimum value of the desired parameter, e.g. the selectivity, is achieved. Values of from 0.1% by weight to 30% by weight are preferred. However, in realistic industrial operation, the comparatively high costs of some surfactants have to be taken into account. In such a case, the surfactant content is preferably from 0.1 to 2 % by weight, particularly preferably from 0.1 to

0.5 % by weight. The percentages here are based on the total mass of liquid phase.

The multiphase systems occurring in the process of the present invention and comprising at least one hydrophilic ("aqueous") phase and at least one phase which is not completely miscible with water are also referred to as emulsions. The industrially most important emulsions generally comprise a water phase and an organic phase, which is usually referred to as "oil phase" . A distinction is made between the internal or disperse phase and the external or continuous phase. The internal phase is, in this sense, the part of the system which is present in small droplets. Each such system can exist in two different forms in which water or oil can be the internal phase. Accordingly, a distinction is made between water-in-oil and oil-in- water emulsions. The present invention applies to all binary liquid systems which are known to those skilled in the art and are not completely miscible, including the abovementioned emulsions and in particular micro- emulsions .

In the case of the system described in the present invention, which comprises a heterogeneous catalyst and also a liquid substrate phase and a cosolvent which is at least partially immiscible therewith, it can, for example, be useful for the aqueous phase, here the cosolvent, to be preferentially adsorbed on the heterogeneous catalyst and possibly block active centres. Such an effect can be of assistance in controlling the selectivity and it may be expected that the use of additives could influence the interfaces in question so as to increase the activity and/or selectivity.

A possible embodiment of the substrate (possibly with solvent) /cosolvent system is a microemulsion. This is a macroscopically homogeneous, optically transparent, low-viscosity, thermodynamically stable mixture of two liquids which are immiscible with one another using at least one nonionic or ionic surfactant. If an ionic surfactant having only a hydrophobic radical is added to the two mutually insoluble components, a cosurfactant is additionally needed to form the microemulsion. Short-chain aliphatic alcohols are preferred here. Due to their extremely large surface area per unit volume, microemulsions offer an ideal "solvent" for chemical reactions in which mass transfer through interfaces plays a role.

For the purposes of the present invention, a salt is any at least partially charged compound, i.e. any compound which is not neutral in terms of its charge distribution. Ionic compounds which are at least partially soluble in the aqueous phase and break up into anions and cations which may be partially solvated are particularly preferred. Metal salts are more preferred and metal salts having at least doubly charged cations are further preferred.

Since the addition of salts particularly affects interfaces, the amount of salt added can be kept comparatively small. In particular, it would not be expected, and this has also been confirmed by experiment, that an increase in the salt concentration would lead to significant effects. For the purposes of the present invention, a low salt content, i.e. a salt content of from 10^"7 to 1 mol/1, is preferred. Particular preference is given to a value of from 10^"5 mol/1 to 10^"2 mol/1. Here, the proportions are based on litres of the aqueous phase.

For the purposes of the present invention, the type and proportion of cosolvent and also the type and proportion of the surface-active substance and the type of salt and possibly also other parameters and/or additive are varied in a multidimensional parameter matrix so as to optimize the selectivity and/or the yield.

The process of the present invention for hydrogenating organic substrates comprises bringing the above- described substances, but at least (i) at least one organic substrate, (ii) at least one cosolvent, (iii) at least one surface-active substance and (iv) at least one salt which is at least partially soluble in the liquid phase into contact with a hydrogenating agent. The conditions under which this occurs, the order in which the components are combined and/or brought into contact with one another and the containers are subject to no particular restrictions according to the present invention. The reaction conditions for the hydrogenation of aromatics which are known to those skilled in the art in organic chemistry in general and in industrial organic chemistry in particular are particularly preferred. Further preference is given to the conditions which are known for the selective hydrogenation of aromatics.

The process described is illustrated by the following examples, without these restricting the general validity of what has been said above:

Brief description of the figures:

Figure 1 shows the influence of the use of monools and polyols on the reaction rate in the hydrogenation of toluene (for different proportions of cosolvent).

Figure 2 shows the reaction rates for the hydrogenation of toluene (different proportions by volume of various cosolvents added) .

Figure 3 shows the dependence of the selectivity for the hydrogenation of toluene to methylcyclohexene using lanthanum nitrate as additive.

Figure 4 shows the effect of the combined addition of a salt and a surface-active substance on the selectivity.

Figure 5 shows the effect of adding nonionic, cationic and anionic surfactants at the same mole fraction of zinc salt, respectively.

Figure 6 shows the effect of high proportions of surface-active substances on the olefin yield

Examples:

All the examples described below were carried out in a battery of 8 parallel autoclaves. The autoclaves themselves have a capacity of 60 ml and are designed for a maximum pressure of 200 bar and a maximum temperature of 200°C.

The autoclaves are equipped with four-blade stirrers . The stirrer speed can be varied in a range from 50 to 2 000 revolutions per minute. The experiments described here were carried out at 1 200 rpm. The autoclave can be cooled or heated within a temperature range from 20°C to 200°C. The autoclaves comprise the actual reaction vessel, an upper part (lid) and a sleeve which can heat or cool the reaction vessel. A rubber seal between the reaction vessel and lid ensures that the apparatus is gastight.

Hydrogenation of toluene was carried out by way of example and the conversions and selectivities were determined by gas chromatography. The individual components were weighed directly into GC vials. This was followed by mixing of the sample and then reaction of the substrate with hydrogen gas. At the end of each reaction, samples were taken and these were analysed with the aid of a gas chromatograph . Chromatograms enable to determine both the proportion of the substances prior to the reaction and also conversion, selectivity and yield of the reaction itself.

As catalyst, use was made of solid Ru on carbon (5% Ru; from Aldrich) .

The following surface-active substances were used in the experiments:

Surfactant 1: Polyethylene-block-poly (ethylene glycol): molar mass 920 g/mol; 20% of ethylene oxide in the monomer;

Surfactant 2: Polyethylene-block-poly (ethylene glycol): molar mass 575 g/mol; 20% of ethylene oxide in the monomer ; Surfactant 3: Poly (ethylene glycol) 4-nonylphenyl 3-sulphopropyl diether;

Surfactant 4: Polyethylene-block-poly (ethylene glycol): molar mass (M) 875 g/mol; 20% of ethylene oxide in the monomer;

Surfactant 5: Poly (ethylene glycol) monolaurate: (M: 400 g/mol) ;

Surfactant 6: Poly (ethylene glycol) monolaurate: (M: 600 g/mol) ;

Surfactant 7: Hexadecyltrimethylammonium acetate;

Surfactant 8: Hexadecyltrimethylammonium nitrate.

Example 1 : Hydrogenation of toluene over an Ru/C catalyst using a cosolvent

In this example, the influence of the chain length of the cosolvent at various proportions of cosolvent was examined for methanol, ethanol and 1-propanol. As mentioned above, water was used as comparative system. (Reaction conditions: hydrogenation pressure 70 bar; temperature 60°C; total volume 30 ml; volume ratio of toluene/cosolvent variable; Ru/C 25 mg) .

Figure 2 shows the reaction rates obtained when different proportions by volume of each of the various cosolvents were added. The shape of the reaction rate versus proportions by volume curve is virtually identical for methanol, ethanol and 1-propanol: as the proportion of cosolvent increases, an increase in the reaction rate is observed until finally the maximum reaction rate is achieved at about 50% by volume. Since it is more economical in practice to employ relatively small proportions of cosolvent, a proportion of cosolvent of 30% by volume was chosen for the experiments described below.

Example 2: Selectivity of the hydrogenation of toluene when using an Ru-C catalyst system, water as cosolvent and with addition of a salt (comparative example)

Figure 3 shows the dependence of the selectivity on conversion in the hydrogenation of toluene to methylcyclohexene using lanthanum nitrate as additive, with the indicated molar amount of nitrate being based on the metal (reaction conditions: hydrogenation pressure 70 bar; temperature 60°C; total volume = 30 ml; volume ratio of toluene/water 7:3; Ru/C 25 mg) .

The selectivity to methylcyclohexene can be increased by addition of a salt. A similar shape of the selectivity versus conversion curve was found in each case: as the degree of conversion increases, the selectivity to methylcyclohexene drops, but the selectivity at a conversion of 20% is in each case increased from 1% to 2% when using lanthanum nitrate compared with the use of water as cosolvent without salt. In comparison, the selectivity when zinc nitrate is added is about 1.3%.

This behaviour in respect of the selectivity can be explained by the "salt effect". Since the ionic additives are adsorbed on sites which are particularly hydrogenation active, the intrinsic reactivity of the catalyst is reduced so that the reaction rate drops. In addition, the adsorption enthalpy of toluene and methylcyclohexene is decreased. By means of this weakening of the adsorption, the addition of the salt prevents readsorption of methylcyclohexene on the catalyst and thus also prevents the hydrogenation of methylcyclohexene to methylcyclohexane . The higher charge density of the trivalent ion and the resulting stronger adsorption of lanthanum compared with zinc on the surface of the catalyst produces a further increase in the selectivity.

As may be expected in the case of an interface effect, the amount of salt added plays no significant role.

Example 3: Effect of the simultaneous addition of salt and of surface-active substance on activity and selectivity in the hydrogenation of toluene (process of the invention)

Figure 4 shows the effect of the combined addition of a salt and a surface-active substance (here an anionic surfactant) on the selectivity. The reaction conditions were: hydrogenation pressure 70 bar; total volume = 30 ml; volume ratio of toluene/water 7:3; amount of Ru/C catalyst: 25 mg.

Figure 4 shows the dependence of the selectivity to methylcyclohexene on degree of conversion for the combination of surfactant 3 and zinc salt. It is found that the combination of poly (ethylene glycol) 4-nonyl- phenol 3-sulphopropyl diether and zinc nitrate has a selectivity-increasing effect, since the yield of methylcyclohexene was able to be increased significantly. The shape of the selectivity/conversion curve is similar to that found above when using the salt alone: a decrease in the selectivity to the olefin is found as the degree of conversion increases.

Example 4: Comparison of anionic, cationic and nonionic surfactants in the presence of zinc salts for increasing the selectivity of the hydrogenation of toluene (process of the invention)

A comparison of nonionic, cationic and anionic surfactants at the same mole fraction of zinc salt is shown in Figure 5. The anionic surfactant obviously has a positive influence on the formation of methylcyclo- hexane, while nonionic and cationic surfactants at the same proportions by weight produce only a small improvement at the same surfactant concentration. It should also be mentioned that the reaction rates when zinc salt is added and when a combination of surfactant and zinc salt is added according to the invention are of the same order of magnitude. Comparison of the olefin yields therefore enables direct conclusions to be drawn in respect of the space-time yield.

Example 5 : The process of the invention at high proportions of surface-active substances

Figure 6 shows the effect of high proportions of surface-active substances on the olefin yield. In comparison with small concentrations of surface-active substances, significantly higher olefin yields were able to be achieved when relatively large amounts of surface-active substances were added. The additional addition of lanthanum nitrate produced a further increase in the selectivity.

The maximum olefin yields obtained were 3% of methylcyclohexene at a conversion of 18% with addition of 5 per cent by weight of surfactant 1 and 10^~3 mol of lanthanum nitrate.

Claims

1. Process for hydrogenating at least one organic substrate in at least two liquid phases which are at least partially immiscible with one another using a catalyst, characterized in that at least one of the two or more liquid phases contains at least the following further components: (i) at least one surface-active substance and (ii) at least one salt which is at least partially soluble in at least one of the at least two liquid phases.

2. Process according to Claim 1, characterized in that the hydrogenation of at least one organic substrate is a selective hydrogenation.

3. Process according to Claim 1 or 2 , characterized in that the catalyst is solid.

4. Process according to at least one of the preceding claims, characterized in that the organic substrate or substrates has/have at least one C-C double bond.

5. Process according to Claim 4, characterized in that the organic substrate contains an aromatic or a fused aromatic.

6. Process according to at least one of the preceding claims, characterized in that gaseous hydrogen is used for the hydrogenation.

7. Process according to at least one of the preceding claims, characterized in that one of the two or more liquid phases contains at least one organic substrate or at least one organic substrate dissolved in a nonaqueous solvent.

8. Process according to at least one of the preceding claims, characterized in that at least one phase of the two or more liquid phases is a cosolvent selected from the group consisting of water, monools, polyols and mixtures of at least two of these substances.

9. Process according to at least one of the preceding claims, characterized in that the ratio of solvent to cosolvent is from 10:1 to 1:10.

10. Process according to at least one of the preceding claims, characterized in that the surface-active substance or substances is/are selected from the group consisting of nonionic surfactants and ionic surfactants .

11. Process according to Claim 10, characterized in that the surface-active substance or substances is/are selected from the group consisting of compounds containing at least one ether group.

12. Process according to at least one of the preceding claims, characterized in that the total content of surface-active substances ranges from 0.1% by weight to 30% by weight.

13. Process according to at least one of the preceding claims, characterized in that at least one salt contains a divalent or higher-valent metal ion.