PROCESS FOR THE PREPARATION OF MELAMINE

The invention relates to a process for the preparation of melamine starting from hydrogen cyanide and ammonia in the presence of a catalyst. It is well known that melamine can be prepared from urea at elevated temperature according to the following reaction equation:

6H ₂ N-CO-NH ₂ → C ₃ N ₃ (NH ₂ )S + 6NH ₃ + 3CO ₂

This reaction is strongly endothermic.

There are two basic types of industrial melamine production processes using urea as the raw material, namely, catalytic low-pressure processes and high-pressure processes in which no catalyst is used. In the former, the reactor pressure is approximately 1 MPa or lower, in the latter reactor pressure is usually higher than 8 MPa (Ullmann's Encyclopedia of Industrial Chemistry, 5th edition. Vol. A 16, p. 174-179).

A low pressure catalytic process for preparation of melamine starting from urea is for example known from GB-A 910, 198, teaching that urea is heated in presence of ammonia in a fluidized bed of a boron phosphate or aluminum phosphate catalyst.

A high pressure catalytic process starting from urea is described in GB-A 754,720, disclosing, that urea is reacted preferably in the absence of added ammonia in presence of a catalyst comprising non-precious metals like chromium, zinc, tin and copper. An alternative route for the preparation of melamine uses hydrogen cyanide and ammonia as starting material. Compared to the processes employing urea as starting material a totally different chemistry is involved. Such a process is known for example from GB-A 803,195, where melamine is produced from a vapor mixture of hydrogen cyanide and ammonia, at substantially atmospheric pressure in the presence of a dehydrogenation catalyst which comprises an oxide of chromium and/or a metal of Groups 8-10 of the Periodic Table of the Elements. Examples given of the Group 8-10 metals are platinum,

palladium, nickel and cobalt. A disadvantage of a process such as described in GB-803.195 is the low yield that can maximally be obtained.

Another type of process for the production of melamine from hydrogen cyanide is known for example from GB-A 766,925, where cyanamide and/or melamine are produced from a vapor mixture containing hydrogen cyanide and oxygen over a catalyst at elevated temperatures of at least 350 ⁰ C. The catalyst in GB-A 766,925 is an inert adsorptive material having a surface area in the range of 180-650 m ² /g. It is described there that ammonia can be present when desired. However, the obtained selectivities in GB-766.925 are rather low. It is the object of the invention to reduce or even overcome the above mentioned disadvantages. Therefore it is an object of the invention to provide a process for the production of melamine starting from hydrogen cyanide and ammonia with an increased yield and/or selectivity compared to the state of the art. This object has been attained by a process for the preparation of melamine which process comprises at least the following steps: a. feeding ammonia and hydrogen cyanide, either separately or combined, to a first reactor that contains a catalyst that comprises at least one element selected from group 11 of the Periodic Table of the Elements and which reactor is maintained at a pressure of at least 0.2 MPa and a temperature of 300-500 ⁰ C, at a space velocity lying between 10 ² and 10 ⁶ ml/(g.hr), whereby a gaseous mixture (a) is formed comprising at least melamine, b. optionally purifying mixture (a), producing a melamine containing mixture (b), c. separating at least a part of the formed melamine thereby forming mixture (c) and separated melamine

In the process according to the invention ammonia and hydrogen cyanide are fed to the reactor by generally known means and apparatus. Ammonia and hydrogen cyanide can be combined before they are fed to the reactor, however they can also be fed separately to the reactor. The amount of ammonia and hydrogen cyanide in the feed stream to the reactor can come from new supply of the materials, however it is also possible to use at least a part of mixture (c) and recycle this part of mixture (c) to the reactor. When the recycle is

used, the used ammonia and hydrogen cyanide should be replenished by a new supply of those materials.

The ammonia and hydrogen cyanide as feed to the reactor can be pre-heated before entering the reactor or they can be used as such. When recycled ammonia and hydrogen cyanide is used this can be used to pre-heat the feed stream to the reactor.

The pressure in the reactor is at least 0.2 MPa or higher. It is preferred to use a pressure in the reactor of at least 0.5 MPa, more preferably the pressure is at least 1 MPa. It has been found that at high pressures the yield of melamine is much higher than at low pressures, such as for example atmospheric pressure. Here and hereinafter the pressure is used as the absolute pressure.

The pressure in the reactor is preferably kept below 20 MPa, more preferably below 10 MPa. At very high pressures the construction of the reactor and other apparatus tends to be very heavy, increasing the investments costs considerably. Therefore it is preferred to use lower pressures. A suitable range for the pressure in the reactor is 0.2-20 MPa. A preferred pressure in the reactor lies within the range of 0.5-10 MPa, more preferably 0.5-5 Mpa, most preferred 1-5 MPa.

The temperature in the reactor is maintained between 300 and

500 ⁰ C. Preferably the temperature is kept between 350 and 450 ⁰ C (borders included), because it has surprisingly been found that within this range the yield and selectivity to melamine are most favorable.

The reaction is carried out at a space velocity lying between 10 ² and 10 ⁶ millilitre of reaction mixture per gram of catalyst per hour (ml/(g.h))

(borders included). It was found that the occurrence of undesired side -reactions was minimised at higher space velocities. The space velocity is preferably at least

3.10 ² ml/(g.h), more preferably at least 1.10 ³ ml/(g.h. It was found that the melamine yield was maximised when the space velocity is at most 10 ⁶ ml/(g.h).

Preferably the space velocity is below 3.10 ⁵ ml/(g.h), even more preferably the space velocity is below 10 ⁵ ml/(g.h) The catalyst that is used in the process according to the invention comprises at least one group 11 element. With "group 11 element" is meant an element from group 11 of the Periodic Table of the Elements. A current Internet reference for the IUPAC Periodic Table of Elements is www.iupac.orq/reports/periodic table; the version as used here and hereinafter is

dated June 22, 2007. Within the context of the present invention, the term "group 11 element" is understood to mean the element itself, a compound comprising the element or a mixture of the element and/or compound comprising the element. The catalyst will generally comprise a support material and at least one catalytic active element. The support material is used to increase the catalyst surface area and also for reasons of ease of handling and sturdiness. It is preferred to use the group 11 element supported on at least one heat resistant inorganic material including carbon containing supports, more preferably the support is made from a heat resistant inorganic material with a high surface area. Within the context of the present invention, the term 'support' is understood to mean one heat resistant inorganic material or a mixture of two or more heat resistant inorganic material. Examples of suitable heat-resistant inorganic materials are alumina, silicon carbide or other carbon-containing supports, silica, silica-alumina, titanium oxide, zirconium oxide, chromium oxide, silica magnesia, magnesium oxide, diatomaceous earth, zeolite, pumice, zirconium oxide, cerium oxide, calcium sulphate, titanium phosphate, silicon phosphate, activated carbon and mixtures of any of them. A preferred support material is chosen from the group comprising silica, alumina, silica-alumina, titanium oxide, zirconium oxide, chromium oxide, magnesium oxide, activated carbon and mixtures of any of them.

When one group 11 element is present in the catalyst, the amount of it generally lies within the range 0.5-50 wt%. Preferably the amount of group 11 element lies within the range 1-35 wt%, more preferably 1-25 wt%. The preferred group 11 element is copper. Preferably copper is present in an amount of between 1 and 25 wt%.

When more than one group 11 element is present in the catalyst, the different group 11 elements can be present on or in the same support or on or in different supports. The amount of these active components ( i.e. components showing catalytic activity, to the total weight of the catalyst) varies, depending amongst others upon the support used, method of catalyst preparation and atom ratio of the active components, but is generally at least 0.1 and generally at most 99 wt%. More preferably the amount of active components is at least 1 wt%. Preferably the amount of active components is at most 95 wt%, more preferably at

most 90 wt%, even more preferably at most 80 wt% and most preferably at most 70 wt%.

Another preferred catalyst for use in the process according to the invention comprises in addition to the at least one group 11 element, at least one other element from the transition metals, the alkali metals, the alkaline earth metals and/or the lanthanide group. The transition metals here referred to are considered to be the elements in the groups 3-12 of the Periodic Table of the Elements. The alkali metals are considered to be the elements in group 1 of the Periodic Table of the Elements. The alkaline earth metals are considered to be the elements in group 2 of the Periodic Table of the Elements. The elements from the lanthanide group are considered to be the elements with atomic number 57 (Lanthanide) up to and including 71 (Lutetium).

If in addition to the group 11 element(s), at least one other of transition metal, alkali metal, alkaline earth metal or element from the lanthanide group is present in the catalyst, they can be present on or in the same support or on or in different supports. The amount of these active components ( i.e. components showing catalytic activity, to the total weight of the catalyst) varies, depending amongst others upon the support used, method of catalyst preparation and atom ratio of the active components, but is generally at least 0.1 and generally at most 99 wt%. Preferably the amount of these active components is at least 1 wt%, Preferably the amount of these active components is at most 95 wt%, more preferably at most 90 wt%, even more preferably at most 80 wt% and most preferably at most 70 wt%.

Examples of suitable transition metals are titanium, vanadium, chromium, molybdenum, manganese, ruthenium, cobalt, nickel, palladium and platinum. Preferred transition metals are vanadium, ruthenium, cobalt, nickel, palladium, most preferred transition metals are vanadium, cobalt. Examples of suitable elements from the alkali metals are lithium, potassium, rubidium and cesium, preferred elements are lithium and cesium, most preferred is lithium. A suitable element from the alkaline earth metals is calcium. Examples of suitable elements from the lanthanide group are lanthanum, cerium, praseodymium, ytterbium and lutetium. Preferred elements are cerium and praseodymium, most preferred is praseodymium.

A specially preferred catalyst comprises in addition to copper another element from group 11 , a transition metal and/or an alkali metal and/or an element from the lanthanide group. A suitable catalyst was found to be a catalyst that comprises in addition to the support: copper, vanadium, silver, praseodymium, lithium and cobalt.

The catalyst for use in the process according to the invention preferably contains the group 11 element in an amount higher than the individual amounts of the other components showing catalytic activity. The at least one group 11 element is thus preferably the major (by weight) component showing catalytic activity.

The catalyst to be used in the process according to the invention may be prepared by methods known as such to the skilled person. Reference can for example be made to the "Encyclopedia of Catalysis", John Wiley & Sons, ed. Istvan Horvath, 2007. It has been found advantageous to pre-treat the catalyst before the reactants are fed to the catalyst. A suitable method is to pre-treat the catalyst in a flow containing hydrogen at an elevated temperature during an extended period of time.

The catalyst as used in the process according to the invention can have various shapes, such as for example small particles, granules, wires or gauzes. If the catalyst comprises or consists essentially of particles- either as such or in agglomerated or sintered form- it is preferred that the said particles are in size between 100 nm and 5 mm. The term size is defined herein as the average value of the largest and smallest dimension of a particle. Since the catalyst is essentially in the solid phase and the reaction mixture is essentially in the gaseous or supercritical phase, it follows that the catalyzed reaction step in the process according to the invention falls into the category of heterogeneous catalytic reactions.

It has been found that it is advantageous to perform the process according to the invention in the absence of added oxygen. When no stream of oxygen-containing gas is fed to the reactor the selectivity is better, purification of the products is easier and the process is economically more advantageous as no valuable feedstock is lost in the conversion to unwanted, oxygen-containing, products. With "no stream of oxygen-containing gas is fed to the reactor" is meant

that oxygen is not intentionally fed to the reactor, however low amounts of oxygen can, unintentionally, enter the reactor for example as contamination in the feed stream or by tiny leakages in the apparatus. These low amounts of oxygen will not negatively influence the reactions taking place. It is preferred to keep the level of oxygen in the reactor below 2 vol%, more preferably below 1 vol%, most preferably below 0.5 vol%. It is even more preferred to perform the process according to the invention in the absence of oxygen.

The process for the preparation of melamine according to the invention makes use of a feed of ammonia and hydrogen cyanide to the reactor. Hydrogen cyanide can be bought as such from an external source; however transportation brings a lot of (health) risks with it. Therefore it would be an advantage to be able to synthesize the hydrogen cyanide in-situ, at the same location as where it will be used in further synthesis. For example hydrogen cyanide can be made from NaCN. The present invention will now be elucidated by the following non- limiting examples.

EXAMPLES

General experimental procedure A reactor of diameter 6.35 mm was packed with a pelletized catalyst with typical sieve fraction of 0.5 to 1 mm (for catalyst 1 and 2) or 0.5-1.4 mm (for other catalysts). The solid catalyst bed was packed into the reactor so that it is centered at the part of the reactor which will be in the centre of the reaction oven. The catalyst bed was held in place with quartz wool bungs at either end, and the reactor below the catalyst bed was packed with SiC (0.25 mm fraction) to prevent downward movement of the catalyst. The SiC was held in the reactor with a quartz wool bung at the base of the reactor.

The reactor was placed into the reaction setup and the catalyst bed was pre-treated with H ₂ at a flow of 50 ml/(g.h) for about 12 hours at 500 ⁰ C. The oven temperature was increased to 500 ⁰ C at 5 ^C C per minute and after pre- treatment adjusted to reaction temperature.

Following the pre-treatment, the gas flow was switched to helium to flush the H ₂ out of the system. The system was pressurized using a back pressure valve.

The start of the reaction was marked by shutting the helium valve and opening the valves to the reaction gases (HCN and NH ₃ ) simultaneously. Reaction gas flows were controlled using high pressure mass flow controllers. In some experiments an ISCO pomp was used for NH ₃ dosing. The reactor outlet flow composition was monitored by constant online MS and regular online GC sampling. Reaction duration was at least 4 hours.

At the end of the reaction duration, the reactant gases were switched off, and N ₂ was flushed through the system while decreasing the temperature to room temperature and releasing pressure. After opening the reactor system, the SiC and catalyst were subsequently removed from the reactor and separately extracted with water. Both samples were analysed by liquid chromatography with UV detection on melamine concentration. Melamine yield (given in mol %) is defined as the amount of carbon as fed to the reactor in the form of HCN to have reacted into melamine.

Preparation of the catalysts Catalyst 1 :

120 g of silica (Aerosil type OX380) was, under continuous stirring, added to 1500 g of distilled water so as to obtain a dispersed solution with the silica (dispersed solution I).

136.42 g of copper nitrate trihydrate, 0.58 g of silver nitrate and 2.04 g of cobalt nitrate hexahydrate and 5.25 g of praseodymium chloride hexahydrate were added to 1000 g of distilled water. The pH was adjusted to 1.9 by drop-wise addition of concentrated nitric acid so as to obtain a homogeneous solution. This homogeneous metal solution was added to the dispersion with silica (dispersed solution I) under continuous stirring, resulting in a dispersed solution II.

3.07 g of ammonium meta- vanadate were added to 50 g of distilled water so as to obtain a dispersed vanadate- solution. This dispersed vanadate- solution was added to the silica containing dispersion (II) under continuous stirring, resulting in a dispersion (IV). Finally, 28 g of lithium hydroxide were added to 100 ml of distilled water so as to obtain a homogeneous Li-solution and this Li-solution was added to the silica containing dispersion (IV) under continuous stirring.

This final solution was stirred for one hour and then filtrated over a paper filter with a Buchner funnel. First the wet cake was dried at 100 ⁰ C in a stove and then calcined at 310 ⁰ C during 16 hours. The resulting material was sieved and the fraction of 0.5 - 1.0 mm was collected. The BET surface of this material was 60 m ² /g and the pore volume was 0.77 g per ml. ICP analysis (wt%): Cu(18.0), V(OJ) ₁ Ag(0.1 ), Pr(1.1 ), Co(0.1 ), Li(1.5). The specific surface area and the pore volume were determined by N ₂ adsorption and capillary condensation according to the BET method as described in S. Brunauer, P. H. Emmett, E. Teller, J. Am. Chem. Soc. 60, 1938, 309.

Comparative Catalyst 2:

A catalyst was prepared with the following composition, based on weight percentage:

MgO (Janssen Chimica 97%) was impregnated via incipient wetness impregnation with an aqueous solution of Pd(NOs) ₂ (Merck, 99%) in deionised water. The catalyst was dried in an oven at 80 ⁰ C and then calcined in flowing air (1OnIJh), heated (2°C/min) to 350 ⁰ C and kept at this temperature for 2 hours, resulting in a catalyst comprising 0.5 wt.-% Pd on MgO.

Catalyst 3:

A copper catalyst supported on silica with 20.9 wt.-% copper was prepared via the deposition-precipitation method, according to the following procedure:

6.0 g copper nitrate trihydrate and 4.4 g urea, which is three times the amount required to completely precipitate the copper ions, were dissolved in 500 ml demineralized water. Subsequently 6.3 g silica (Aerosil 200, Degussa, specific area 200 m ² /g) was suspended in this solution. The pH was adjusted at 3.0 using nitric acid. Then the temperature was raised to 9O ⁰ C to start the hydrolysis of urea. After 8h30 (pH=6), the solution was filtered hot and the wet cake was washed with water, dried in air at 100 ⁰ C (1 °C/min) during 6 hours and calcined at 450 ⁰ C (1°C/min) during 16 hours.

Catalyst 4:

A copper-lithium catalyst supported on silica with 25.1 wt% copper was prepared via the precipitation method, according to the following procedure: 12.08 g of silica (Aerosil type OX380) was, under continuous stirring, added to 15Og of distilled water so as to obtain a dispersed solution (dispersed solution I).

13.657 g of copper nitrate trihydrate was added to 100g of distilled water. This metal solution was added to dispersed solution I, under continuous stirring, resulting in a dispersed solution II.

2.81 g of lithium hydroxide was added to 10 ml of distilled water so as to obtain a homogeneous solution, which was added to the dispersion (II) under continuous stirring.

This final solution was stirred for one hour and then filtrated over a paper filter with a Buchner funnel. The wet cake was washed with water, dried at 100 ⁰ C (1°C/min) in a stove and then calcined at 310 ⁰ C (1 °C/min) during 16 hours. The resulting material was sieved and the fraction of 0.5 - 1.0 mm was collected.

Catalyst 5:

A copper-lithium catalyst supported on alumina with 20 wt% copper was prepared via the precipitation method, according to the following procedure:

12.01 g of alumina (Harshaw 0104) was, under continuous stirring, added to 150g of distilled water so as to obtain a dispersed solution (dispersed solution I).

13.653 g of copper nitrate trihydrate was added to 100g of distilled water. This metal solution was added to dispersed solution I, under continuous stirring, resulting in a dispersed solution II. 2.81 g of lithium hydroxide was added to 10 ml of distilled water so as to obtain a homogeneous solution, which was added to the dispersion (II) under continuous stirring.

This final solution was stirred for one hour and then filtrated over a paper filter with a Buchner funnel. The wet cake was washed with water, dried

at 100 ⁰ C (1 °C/min) in a stove and then calcined at 310 ⁰ C (1 °C/min) during 16 hours. The resulting material was sieved and the fraction of 0.5 - 1.0 mm was collected.

Catalyst 6:

A gold catalyst supported on titanium oxide with 1.5 wt% gold was provided by the

World Gold Council. http://www.utilisegold.com/uses applications/catalysis/reference catalysts/

Catalyst 7:

A silver-lithium catalyst supported on alumina with 25 wt% silver was prepared via the precipitation method, following the same procedure as catalyst 5, but with 12.01 g alumina, 5.3 g silver nitrate and 0.79 g lithium oxide.

Example 1

The plug flow reactor was filled with 0.24 g of the pelletized Cu-V- Ag-Pr-Co-Li/Siθ ₂ catalyst as prepared above (catalyst 1 ). After catalyst pre- treatment in H ₂ , cooling down to the desired reaction temperature of 400 ⁰ C and pressurizing to 5 MPa, the reactant gases were led through the reactor with a total gas hourly space velocity of 910 ml/(g.h). The inlet gas flow composition amounted to 0.1 vol% HCN, 36.5 vol% NH ₃ and 63.4 vol% He. Of the amount of carbon as fed to the reactor in the form of HCN, 4.7 mol-% was found to have reacted into melamine. Of all melamine formed, more than 99% was found on the SiC, i.e. the cooler part below the catalyst and less than 1 % was extracted from the catalyst. Selectivity to melamine was 98%.

Example 2

The general procedure as described for example 1 was followed, except that the reactor was packed with the double amount of catalyst, reducing the gas hourly space velocity to 500 ml/(g.h). A melamine yield of 12.3 % was obtained.

Example 3

The same settings were applied as in Example 2, except that the reactor was operated at the lower pressure of 4 MPa. A melamine yield of 10.6 % was obtained.

Example 4

The same settings were applied as in example 2, except that the reactor was operated at a lower temperature of 380 ⁰ C. A melamine yield of 4.6 % was obtained.

Comparative Experiment A

The same settings were applied as in Example 3, except that no catalyst was packed in the reactor. A melamine yield of only 0.13% was obtained.

Comparative Experiment B

The same settings were applied as in Example 1 , except that the reactor was packed with comparative catalyst 2: Pd-MgO catalyst. A melamine yield of only 0.32% was obtained

Comparative Experiment C

The same settings were applied as in Example 1 , except that the reactor was operated at atmospheric pressure. A melamine yield of only 0.18% was obtained.

Examples 5 - 9 and Comparative Examples D and E

In Examples 5 and 6 and in Comparative Example D Example 1 was repeated at a pressure of 5 MPa, a temperature of 400 ⁰ C, with a GHSV of 500 ml/(g.h). The mixture of reactants contained 10 vol% HCN and 90 vol% NH ₃ . In Examples 7, 8 and 9 and in Comparative Example E,

Example 1 was repeated at a pressure of 3 MPa ₁ a temperature of 400 ⁰ C, with a GHSV of 1200 ml/(g.h). The mixture of reactants contained 0.13 vol% HCN and 57.8 vol% NH ₃ .

The results are summarized in Table 1.

Table 1 :

a ⁾ Yield is calculated on basis of C introduced by HCN.