Hydrogen generating system and hydrodehalogenation method

The main object of the present invention is a hydrogen generator system. It also relates to a process for generating hydrogen and its applications, in particular in a process for hydrodehalogenation of halogenated organic compounds present in aqueous media to be purified.

The present invention relates more particularly to the catalytic degradation of pollutants, more particularly to volatile halogenated organic compounds known as COHVs such as perchlorethylene (PCE), vinyl chloride (CV), dichloroethane (DCE), dichlorethylene, chloroform, carbon tetrachloride, and trichlorethylene (TCE) ...

Conventionally, this type of degradation is carried out according to a process which generally involves a reduction of these pollutants using hydrogen, and this in the presence of a so-called hydrodehalogenation catalyst, generally based on palladium fixed on an inorganic support. . More precisely, a reductive dehalogenation of the halogenated components is carried out during which the halogen is extracted from the molecule in the form of a free halogenated ion in aqueous solution and replaced on the molecule by a hydrogen ion. This type of reaction therefore requires a source of electrons.

Two alternatives are currently available for carrying out this type of reaction.

According to a first variant, a zero-valent metal, preferably iron, is used as the source of electrons and source of metal for the dehalogenation. This approach has the advantage of being relatively inexpensive but on the other hand has the drawbacks of being not very fast and of not being suitable for all VOCs and in particular for vinyl chloride.

The second alternative implies for its part the use of a more elaborate and therefore more expensive catalytic system, which also requires the parallel use of a continuous source of hydrogen. Generally, this hydrogen is introduced in the form of gaseous hydrogen or generated in situ using a complex such as hydrazine or borohydrides.

The object of the present invention is more precisely to propose a third alternative to the two alternatives mentioned above and which is based more particularly on the use of an original hydrogen generator system.

Unexpectedly, the inventors have demonstrated that it was possible to give satisfaction simultaneously in terms of cost and kinetics, using a hydrogen generating system which notably involves the use of a hydrogen metal. zero valence like iron, as a source of electrons.

More precisely, the first object of the present invention is a hydrogen generator system, characterized in that it associates with a metal, corrodible by water, an inorganic material, said material having a specific surface area suitable for the fixation of the gas. or oxide and / or hydroxide forms of said metal generated during its corrosion.

Advantageously, the association of an inorganic material with the metal considered as an electron source makes it possible, in fact, to significantly increase the quantity of hydrogen generated compared to a conventional process, that is to say not using than the zero valence metal.

As for the zero-valent metal, it has a sufficiently negative Redox potential that it can reduce water.

By way of representative of metals suitable for the invention, mention may more particularly be made of steel, iron, zinc, aluminum, tin, bismuth, cobalt and nickel.

Preferably, it is zero-valent iron which is particularly advantageous in view of its low cost.

As regards the inorganic material, it is preferably chosen from metal oxides, mixed or not, insofar as they are of course inert under the reaction conditions.

By way of representative of these oxides, mention may more particularly be made of aluminas, silicas, oxides of zirconium, cerium, titanium, iron and zeolites.

Inorganic materials can be used in different forms: powder, shaped products such as granules (for example 5 cylinders or balls), pellets, monoliths (blocks in the shape of honeycombs) which are obtained by extrusion, molding, compacting or any other type of known process. In practice, industrially, it is the forms of granules, beads or monoliths which present the most advantages both in terms of efficiency and in terms of convenience of use.

The inorganic material preferably has a specific surface area greater than that of the zero valent metal.

Inorganic materials generally have a greater specific surface area of ​​at least a factor of 100, and preferably 10.⁴, to that of the metal, this factor being able to rise up to a value of 10⁶.

According to a preferred embodiment of the invention, it is a synthetic or natural zeolite.

By zeolite is meant a crystallized tectosilicate of natural or synthetic origin, the crystals of which result from the three-dimensional assembly of tetrahedral units of SiO₄ and to₄Where T represents a trivalent element such as aluminum, gallium, boron and iron, preferably aluminum. The aluminosilicate type zeolites are the most common.

Among the zeolites, it is possible to use natural zeolites such as, for example ,offretite, clinoptilotite, erionite, chabazite, philipsite.

Synthetic zeolites are also suitable.

5 As examples of synthetic zeolites with a one-dimensional network, mention may be made, inter alia, of ZSM-4 zeolite, ZSM-12 zeolite, ZSM-22 zeolite, ZSM-23 zeolite and ZSM-48 zeolite.

As examples of zeolites with a two-dimensional network which are preferably used, mention may be made of zeolite beta, mordenite and ferrierite.

Use is preferably made of synthetic zeolites and more particularly of those which are in the following forms:

- mazzite with a Si / Al molar ratio of 3.4,

- zeolite L with an Si / Al molar ratio of 1.5 to 3.5,

- mordenite with a Si / Al molar ratio of 5 to 15,

- ferrierite with a Si / Al molar ratio of 3 to 10,

- Si / Al molar ratio Ibffretite from 4 to 8.5,

- beta zeolites with an Si / Al molar ratio of 15 to 25,

- Y zeolites, in particular zeolites obtained after dealumination treatment (for example hydrotreatment, washing with hydrochloric acid or treatment with SiCl₄), more particularly US-Y zeolites with an Si / Al molar ratio greater than 3, preferably between 6 and 60,

- the faujasite-type X zeolite with an Si / Al molar ratio of 0.7 to 1.5,

- ZSM-5 zeolites or aluminum silicalite with a Si / Al 15 molar ratio of 10 to 2000, and

- ZSM-11 zeolite with a molar ratio of 5 to 30.

Preferably, the inorganic material is a zeolite having a specific surface area greater than 10 m²/ g.

The two zeolites described in the examples below prove in this respect to be particularly advantageous in the context of the present invention.

In fact, without wishing to be bound by a specific mechanistic explanation, it seems that the inorganic material acts as a specific support towards the metal oxides and / or hydroxides generated during the oxidation of the zero valent metal.

5 Indeed, these hydroxides are generated automatically during the reaction of water on the metal according to the following diagram:

M ° -> Mn⁺ + don't^-

2 hours₂O + 2e ^ H₂ + 2OHM + n (H₂O) -> M (OH)_not + ± Hz

By attaching preferentially to the inorganic material and not to the surface of the metal in the state of zero valence, the metal hydroxides generated thus significantly limit the deactivation of the latter.

Thus, in the particular case where the generation of hydrogen 5 is carried out by combining 3 g of iron for 15 g of water, and in the presence of a support such as a zeolite, the production of hydrogen generated on a 24 hour period turns out to be 250 times longer than that generated under the same operating conditions but in the absence of said support.

According to a preferred variant of the invention, the zero-valent metal and the inorganic material are associated in an amount of 0.5 to 40%, and preferably 1 to 20% by weight of said material relative to the weight of the metal.

This adjustment between the two compounds is of course also a function of the specific surface of the inorganic material. It is understood that the required amount of inorganic material is inversely proportional to its specific surface.

According to a preferred variant, the zero-valent metal is not supported by the inorganic material associated with it. It is also conceivable that the inorganic material further serves as a support for another metal capable of acting as a catalyst for a reaction subsequent to or concomitant with the generation of hydrogen.

The claimed hydrogen generator system is particularly advantageous in the field of effluent treatment, for example for the reduction of COHVs and / or nitrates, in the field of the reduction of nitro compounds, especially aromatics. In general, one can in fact consider its use for any application requiring a continuous supply of hydrogen.

The present invention also relates to a process for the generation of hydrogen by reduction of water with the aid of a suitable metal, characterized in that said reduction is carried out in an aqueous medium in the presence of water. 'a sufficient quantity of an inorganic material, the specific surface of which is suitable for the deposition of the oxide and / or hydroxide form (s) of the metal generated during said reduction.

Of course, the inorganic material and the metal meet the definitions presented above in the context of the claimed system.

In the claimed process, the inorganic material and the metal can be directly introduced into the aqueous medium to be treated and the whole is then stirred so as to optimize the conditions for generating hydrogen.

Another variant of the process may consist in circulating the aqueous medium to be treated through a fixed bed comprising at least said metal and the inorganic material.

The present invention is also directed to the use in a process for the generation of hydrogen by reduction of water using a suitable metal, an inorganic material for the purpose of fixing the oxide form (s) and / or hydroxides of said metal generated during the reduction.

The present invention also relates to the application of a hydrogen generation system as defined above for the hydrodehalogenation of volatile halogenated organic compounds in an aqueous medium.

More specifically, it proposes a process for the hydrodehalogenation of volatile halogenated organic compounds present in an aqueous medium, characterized in that it implements a generation of hydrogen by a hydrogen generating system in accordance with the invention 2 And catalytic hydrodehalogenation of the volatile halogenated organic compounds using the hydrogen thus formed and a suitable supported catalyst.

Unexpectedly, the inventors indeed noted that the beneficial effect of the presence of an inorganic material for the generation of hydrogen could moreover be effectively exploited for the hydrodehalogenation. Placing the hydrodehalogenation catalyst in an environment highly enriched in hydrogen allows the hydrodehalogenation kinetics to be considerably increased. This advantageous aspect of the claimed process emerges in particular from the examples presented below.

Advantageously, the inorganic material present in the system is used in an amount such that its developed area (that is to say total) is greater, preferably by at least a factor of 5, than the developed area of ​​the inorganic material constituting the support of the hydrodehalogenation catalyst. Such a developed surface advantageously makes it possible to preserve over time the catalytic performance of the metal making up the hydrodehalogenation catalyst.

As regards the hydrodehalogenation catalyst, it generally comprises, as metal, a metal chosen from palladium, nickel, ruthenium, platinum and / or rhodium. Preferably, it is palladium.

This metal is also supported on an inorganic material. This inorganic material can be chosen from those defined above. Preferably, it is an alumina or a zeolite.

As a more particularly preferred hydrodehalogenation catalyst in the context of the invention, mention may be made of palladium attached to alumina.

It is conceivable that the support to the surface of which the hydrodehalogenation catalyst is attached simultaneously performs the function of the inorganic material involved in the hydrogen generation reaction. According to this variant, it turns out to be possible to significantly reduce the

5 quantity of metal constituting the supported catalyst. The metal constituting the supported catalyst can then be advantageously used at a rate of 10 to 150 mmol / kmole of the zero-valent metal.

Thus, in the particular case where the zero-valent metal is iron and the supported catalyst is palladium / alumina, the hydrodehalogenation process can advantageously be carried out with a palladium / iron mass ratio of less than 100 mg. of palladium / kg of iron against 500 to 5000 mg Pd / kg Iron for conventional processes.

The claimed process can be applied to the reduction of all the organic compounds represented by the families of chlorinated solvents such as trichlorethylene, chlorinated aromatics such as chlorobenzene, 5 chlorophenols or even plant protection products such as Lindane ™, Dinoterbone ™ and nitro-compounds.

Thus, the claimed hydrodehalogenation process can be applied to the purification of groundwater in a temperature range which may vary from 4 to 35 ° C. It can in particular be carried out within a reactor.

According to a preferred variant of the invention, the hydrogen generation system and the hydrodehalogenation catalyst are separated within the reactor. The zero-valent metal and the inorganic material are placed in the lower part of the reactor, at which the liquid medium to be treated is introduced. The hydrogen generated in this lower part then moves towards the upper part of the reactor where the hydrodehalogenation catalyst is placed.

According to this arrangement, the metal oxides and / or hydroxides formed during the generation of hydrogen are preferentially deposited on the inorganic material present in the lower level of the reactor and not on the supported hydrodehalogenation catalyst. In this way, the activity of the hydrodehalogenation catalyst is optimized and its performance is maintained.

5 The examples appearing below are presented by way of illustration and without limitation of the field of the invention.

The supports tested in the examples below are two US-Y zeolites (4% Na₂O₃ with Si / Al of 2.5) (marketed by Engelhard) called 3 0 zeolite A and a HY-CBV 400 zeolite (2.5% Na₂O₃ and Si / Al of 1.5 and a specific surface of 50 m²/ g) (marketed by Zéolyst International) known as zeolite B.

EXAMPLE 1

15 g of water, 3 g of iron and 300 mg of an A or B zeolite are mixed.

After stirring over a period of 24 hours, it is noted that, in each of the tests, the quantity of hydrogen generated is 250 times greater than the quantity of hydrogen generated in a control test, that is to say in the absence zeolite.

It is in fact 16.5 ml / kg water / hour against 0.067 ml / kg water / hour for the control test.

EXAMPLE 2

1.5 mg of trichlorethylene is introduced in the form of a solution at approximately 100 ppm in 15 g of water. Are also introduced into this solution, 3 g of iron and 30 mg of a Pd / Al catalyst₂O₃. After 18 hours 30 minutes of stirring, a degree of conversion of 30% of the trichlorethylene is reached against only a degree of conversion of less than 5% for this same system in the absence of said catalyst.

In a variant of this test, the reaction is carried out in the presence of 300 mg of alumina (CBL alumina marketed by Procatalyse).

Under these conditions, a degree of conversion of 100% is reached in 18 hours 30 minutes. The only reaction product observed is ethane.