The present invention relates to uses of absorbent materials, and to methods relating thereto.

In particular, the present invention relates to absorbent materials which have desirable properties as an absorbent of ammonia. The invention relates especially to the use of such materials in the manufacture of ammonia. The Haber-Bosch process is a primary means for current industrial ammonia production. In this process, hydrogen and nitrogen gases are typically reacted at a high temperature and pressure over a catalyst. The synthesis of ammonia is governed by an equilibrium reaction, whereby the amount of hydrogen and nitrogen converted to ammonia is limited by the equilibrium concentration of ammonia, as described by the following equation:

N ₂ + 3H ₂ < > 2NH ₃ ΔΗ ₂₉₈ = -91 .8 kJ/mol

Accordingly, the equilibrium concentration of ammonia is dependent on the temperature and pressure at which the reaction takes place. An increase in pressure favours the forward reaction (i.e. ammonia production), and therefore commercial processes usually operate at an elevated pressure, for example from 150 to 250 bar. By contrast, an increase in temperature favours the reverse reaction. However, whilst a lower temperature produces a higher equilibrium concentration of ammonia, the rate of the reaction is inversely dependent on temperature, and therefore at a low temperature the forward reaction is slow. In commercial processes the choice of temperature is often a compromise between achieving a suitable equilibrium concentration of ammonia and a suitable rate of reaction. The operating temperature in commercial processes is typically from 350 °C to 550 °C, and the equilibrium concentration of ammonia is approximately 20 %. Thus, the equilibrium mixture may contain about 80% of unreacted feedstock gas (nitrogen and hydrogen).

Unreacted feedstock gas is often separated from the process stream and recycled back to the reactor. Separation is usually achieved by cooling the process stream to about 5 °C to condense out the ammonia, before the unreacted feedstock gas is re- heated and re-compressed for return to the reactor feed stream. The cooling, reheating and re-compression steps add considerable complexity and costs to industrial ammonia synthesis processes and are energy intensive. Magnesium chloride (MgCI ₂ ) is known for its capacity for absorbing ammonia and has been proposed as an alternative means of separating ammonia from the process stream in the Haber-Bosch process. However, MgCI ₂ has not been used commercially in the Haber-Bosch process. The present invention seeks to provide improved means for preparing ammonia which overcome at least one disadvantage of the prior art.

According to a first aspect of the present invention, there is provided the use of a layered double hydroxide (LDH) material to absorb ammonia in a Haber-Bosch process.

According to a second aspect of the present invention there is provided a method of absorbing ammonia from a Haber-Bosch reaction mixture, the method comprising contacting the reaction mixture with a layered double hydroxide material.

Preferred features of the first and second aspects will now be described.

Layered double hydroxide (LDH) materials are well known materials and their structures and properties will be known to the person skilled in the art. For example, compounds of this type are described by Evans and Slade in "Structural Aspects of Layered Double Hydroxides" in "Layered Double Hydroxides" edited by: X.Duan and D.G. Evans, Volume 1 19 of the series Structure and Bonding, pp 1-87, Springer, 2005. Any of the LDH materials described in this document may be useful in the present invention.

The basic structure of LDH materials is based on that of brucite [Mg(OH) ₂ ]. Brucite consists of magnesium ions surrounded approximately octahedrally by hydroxide ions. The octahedral units share common edges to form infinite layers which stack upon one another to form a three dimensional structure. The basic structure of an LDH material may be derived by substituting a fraction of the divalent cations of a brucite lattice with trivalent cations. The layers thereby acquire a positive charge. Intercalation of anions between the layers balances this charge. Water may also be intercalated. Many different types of LDH materials may be provided by varying the nature of the divalent and trivalent species and the relative quantities thereof.

Preferred LDH materials for use in the present invention include compounds of formula (1 ):

[M |2- +

1-xM ³⁺ _x (OH)2T(A ⁿ -)x/n

(1 )

wherein M ²⁺ is a divalent metal cation, M ³⁺ is a trivalent metal cation, A ^n" is an anion; and x is from 0.001 to 0.99.

M ²⁺ may be selected from any divalent cation. Suitably the divalent metal cation M ²⁺ is selected from Mg ²⁺ , Fe ²⁺ , Co ²⁺ , Cu ²⁺ , Ni ²⁺ , Zn ²⁺ or mixtures thereof. Preferably the divalent metal cation comprises Mg ²⁺ .

M ³⁺ may be selected from any trivalent cation. Suitably the trivalent metal cation M ³⁺ is selected from Al ³⁺ , Cr ³⁺ , Ga ³⁺ , Mn ³⁺ , Fe ³⁺ or mixtures thereof. Preferably the trivalent metal cation comprises Al ³⁺ .

Any suitable anion may be used as A ^n" . Suitable anions will be known to the person skilled in the art.

Preferred anions for use herein include OH ^" , CI ^" , Br ^" , N0 ₃ ^" , C0 ₃ ^2" , S0 ₄ ^2" , Se0 ₄ ^2" or mixtures thereof. x is from 0.001 to 0.99. Preferably x is from 0.01 to 0.90, more preferably from 0.05 to 0.80. In some embodiments x is from 0.07 to 0.50, suitably from 0.20 to 0.33.

As the skilled person will appreciate, the value of n will vary depending on the nature of the anion. Suitably n is from 1 to 4. Preferably n is 1 or 2.

In some embodiments the LDH materials may further comprise neutral molecules that are intercalated between the layers of the material

Suitable neutral molecules include formic acid, methanol, water, nitrophenol or mixtures thereof.

The LDH materials may include water of crystallisation in their structure. Typically, however, at the operating temperatures of the Haber-Bosch process any water of crystallisation will have been driven off.

The inventors have advantageously found that the LDH materials of the present invention may be suitable for use at temperatures of up to 350°C, suitably up to 400°C, suitably up to 450°C, for example up to 550°C. Advantageously the LDH materials used in the present invention exhibit faster rates of absorption of ammonia under typical Haber-Bosch reaction conditions compared with MgCI ₂ particles.

It is believed that the more open structure of the LDH materials due to the interlayer galleries allows an increased rate of diffusion of ammonia into the material. Thus as well as having a high absorption capacity, absorption of ammonia is quickly achieved.

The reactants and products of the reaction mixture in the Haber-Bosch process are in the gaseous phase. Because the LDH materials absorb ammonia, they remove the product from the equilibrium mixture. This drives the equilibrium reaction in favour of the production of ammonia.

Suitably under the same conditions of temperature, pressure and reaction time, the use of a LDH material as an absorbent of ammonia according to the present invention in the Haber-Bosch process increases the yield of ammonia. The LDH materials have a significant effect on the equilibrium of the Haber-Bosch process. The materials are suitably able to absorb ammonia from the reaction mixture at a rate that is fast enough to prevent the equilibrium concentration of ammonia from being reached. This removal of ammonia from the reaction mixture drives the forward reaction, significantly increasing the conversion rate.

According to a third aspect of the present invention, there is provided a method of preparing ammonia, the method comprising admixing hydrogen and nitrogen in the presence of a suitable catalyst and a layered double hydroxide (LDH) material.

The method of the third aspect is suitably a synthesis of ammonia according to the Haber-Bosch process.

Preferred features of the first and second aspects of the invention also apply as appropriate to the method of the third aspect. Further features of the invention as defined in relation to the third aspect may also apply to the first and second aspects.

Hydrogen for use in the method of the third aspect may be provided from a source of methane or natural gas using a process known as steam reforming. Such processes for providing hydrogen, and other processes for providing hydrogen, will be known to a person skilled in the art.

Nitrogen may be provided from the atmosphere. A person skilled in the art will be familiar with methods by which nitrogen may be provided for use in the Haber-Bosch process.

Hydrogen and nitrogen are suitably admixed in an initial molar ratio of from 10:1 to 1 :2, preferably 5:1 to 1 :1 , more preferably from 3.5:1 to 2.5:1 , suitably approximately 3:1 .

A catalyst is used in the method of the third aspect. Any suitable catalyst able to facilitate the reaction of hydrogen and nitrogen in the Haber-Bosch process may be used. Catalysts of this type will be known to the person skilled in the art. Non-limiting examples of suitable catalysts include K ₂ 0, CaO, Si0 ₂ , and Al ₂ 0 ₃ . The method of the third aspect of the present invention is carried out in the presence of an LDH material.

Suitable LDH materials are as defined in relation to the first and second aspects.

Preferably the layered double hydroxide (LDH) material has formula (1 ):

(1 )

wherein M ²⁺ is a divalent metal cation, M ³⁺ is a trivalent metal cation, A ^n" is an anion; and x is from 0.001 to 0.99.

In the method of the third aspect, the reaction mixture is suitably heated under pressure in a reaction vessel.

Typically the reaction mixture may be heated to a temperature of from 200 to 550 °C.

In typical Haber-Bosch processes, the pressure in the reaction vessel is from 150 to 350 bar.

An advantage of the present invention is that ammonia may be prepared at lower pressures. This is because the removal of ammonia by the LDH material drives the forward reaction. This drives the equilibrium in the same way as an increase in pressure. Thus the LDH material can lead to an increase in yield and/or allow a lower pressure to be used.

In some embodiments the pressure used in the method of the third aspect may be less than 100 bar.

In some embodiments the pressure may be less than 80 bar, suitably less than 50 bar, for example less than 20 bar or less than 10 bar.

In some embodiments the method of the third aspect may be carried out at a pressure of 1 -2 bar. Advantageously because the use of an LDH material as an ammonia absorbent drives the equilibrium reaction in favour of ammonia production, the present invention may enable a lower pressure to be used whilst still achieving a comparable or improved yield compared to a process in which the LDH material is not used.

In a typical Haber-Bosch process of the prior art, ammonia is collected from the reaction mixture by condensation. To collect the ammonia the temperature in the reaction vessel is reduced to about 5 °C. This cooling step adds significantly to the costs and complexity of the Haber-Bosch process.

As the equilibrium yield is typically about 20%, when cooled, the ammonia is removed and the reactants which make up 80% of the mixture (plus new additional reactants as appropriate) are reheated in further cycle. Thus, many cooling and reheating steps occur in a typical plant.

Advantageously ammonia may be absorbed by the LDH materials according to the present invention at the operating process temperature. The cooling step therefore may be unnecessary. Thus, a significant cost benefit and/or a reduction in the complexity of the process may be achieved by using the present LDH materials in the Haber-Bosch process.

The LDH materials are contacted with the reaction mixture, which at equilibrium comprises hydrogen, nitrogen and ammonia gases. The LDH materials are solid materials. They are suitably provided in a form which facilitates diffusion of gases into the material.

The LDH materials may be provided in powdered form. The materials may be finely divided and blown throughout the reaction mixture by agitation of the gases within.

Preferably the LDH materials are provided on a solid support which is shaped to facilitate diffusion of the reaction mixture gases into the LDH materials. Means of supporting the LDH material in such a manner will be known to the person skilled in the art.

In some preferred embodiments the LDH materials are packed in a column. Suitably the supported LDH materials are in contact with the reaction mixture.

In some embodiments the LDH materials may be periodically contacted with the reaction mixture.

Preferably the LDH material is continuously contacted with the reaction mixture during the reaction. The concentration of the LDH materials present in the reaction vessel is selected to provide sufficient absorption capacity depending on the specific amounts of reactants and reaction conditions.

The skilled person will appreciate that although removal of ammonia from the reaction mixture will drive the equilibrium in the desired direction, because this is an equilibrium reaction and pressure driven, it is unlikely that the reaction will be driven to completion before adding further reactants.

Because the LDH material is a solid material it can be readily separated from the gaseous reaction mixture. Means for removing the material from the reaction mixture will be known to the person skilled in the art and include for example, filtration.

In some embodiments of the third aspect, the ammonia is prepared in a batch process.

In the method of the third aspect, the LDH material suitably absorbs at least 1 wt% of the ammonia which would have been present in the equilibrium reaction mixture if said material had been absent, suitably at least 5 wt%, preferably at least 10 wt%, suitably at least 30 wt%, for example at least 50 wt%.

In some embodiments the LDH material may absorb more than 70 wt% of the ammonia which would have been present in the equilibrium reaction mixture if said material had been absent, for example more than 80 wt% or more than 90 wt%. Suitably for the same conditions of temperature, pressure and reaction time, the inclusion of an LDH material in the reaction mixture may increase the yield of ammonia by at least 10%, preferably at least 20%, suitably at least 30%. In other embodiments the yield may be maintained but a lower pressure may be used to achieve this yield.

Once the LDH material has been separated from the reaction mixture the ammonia absorbed therein may be desorbed and collected.

Desorption may be achieved by any suitable means. Such means may include, for example, heating the material under reduced pressure and then condensing the ammonia gas released. The regenerated LDH material can then be reused as an absorbent in a subsequent process. This is a further advantage of the invention.

The present invention provides a highly advantageous use and methods in which the efficiency of the Haber-Bosch reaction is improved. The yield of the reaction may be increased and/or the operating pressure may be reduced. This leads to cost savings and environmental benefits.