In a further aspect the present invention relates to an inlet section for feeding reactants to a reactor, such as a microreactor according to the present invention.

State of the art Such a reactor is, for example, disclosed in the article entitled'A novel catalyst testing microreactor for heterogeneous gas phase reactions'by G. Kolb et al., 1MRET VI, 2002, New Orleans, pp. 61-69. This article describes a test reactor for testing multiple plates coated with a catalyst, in parallel or in series. The design is modular and comprises a titanium reactor core provided with drawers by means of which the plates can be pushed into the reactor core. The individual plates are separated from one another by graphite seals and the whole is held together by a steel jacket. The reactor is suitable for temperatures up to 500 °C.

A further microreactor is disclosed in the article entitled'A multiple microreactor system for parallel catalyst preparation and testing'by P. Pantu and G. R. Gavalas, AIChE Journal 48 [4], pp. 815-819,2002. This reactor consists of thin quartz rods that are provided on the surface thereof with a catalyst composition to be tested. The quartz rods are placed in a quartz tube and are held in place by a Teflon sealing ring. Multiple quartz tubes form the reactor. As a result of the use of quartz, reactions at high temperature are possible (700 °C). This reactor has the disadvantage that it is an expensive and fragile construction and that the test rods are difficult to produce, as a result of which the preparation for a series of tests is complex. It is thus not possible to test a large quantity of different catalyst compositions. Furthermore, the reaction conditions in each quartz tube (temperature and flow rate) are not uniform. Furthermore, as a result of the use of quartz, heat removal is not good, as a result of which no highly exothermic reactions are possible in this reactor.

Summary of the invention The aim of the present invention is to provide a microreactor that does not display the disadvantages described above and, in particular, is suitable for testing catalyst compositions (such as zeolites) that are used in highly exothermic reactions that take place at a high temperature (above 400 °C).

According to the present invention a microreactor according to the type defined in the preamble is provided, wherein at least the reaction section of the microreactor is essentially made of a material that is able to withstand temperatures above 500 °C, for example above 700 °C, and that has a good thermal conductivity of more than 50 W/mK, for example more than 100 W/mK. One example of such a material is molybdenum. Because the material is able to withstand high temperatures, highly exothermic reactions can take place in the reaction section. As a result of the good thermal conductivity, localised heat build-up (hot spots) is prevented and essentially isothermal conditions in the reaction section are guaranteed. Molybdenum furthermore has favourable properties (modulus of elasticity of 330 109 Pa and a low coefficient of linear extension of 5 10-6 K-l), which also provide good mechanical stability of the microreactor.

In a further embodiment of the microreactor in question, the material has a protective coating for protection against corrosion, at least on surfaces that can come into contact with the reactants. This enables the microreactor to be used for reactions in a highly corrosive environment (for example caused by oxidation or ammoxidation). The protective coating can, for example, be made of molybdenum silicide (MoSi2) or molybdenum boride (MoB).

These coatings give adequate protection at reaction temperatures of up to 700 °C. As an alternative, the protective coating can comprise a thin coating of hafnium (order of magnitude 10 jjm) which can be provided in a manner known per se by means of electrodeposition. As is known, a hafnium coating gives protection against even higher temperatures (for example 1000 °C). As a further alternative the protective coating comprises a thin coating of oc-alumina, which, for example, can be provided with the aid of atomic layer deposition (ALD). This coating can be used up to temperatures of up to 550 °C. hi a practical embodiment of the microreactor in question, the at least two reaction compartments have an essentially rectangular cross-section perpendicular to a longitudinal direction of the microreactor and in a first direction the at least two reaction compartments are positioned alongside one another perpendicularly to the longitudinal direction of the microreactor. As a result, a microreactor is obtained in which multiple reaction compartments (for example eight) are arranged in the reaction section in a compact manner. The dimensions of each compartment can be, for example, 40 x 10 x approx.

2 mm, which provides adequate reaction space for simultaneous testing of a number of catalyst compositions in different compartments in an exothermic reaction with a conversion of less than 10-15 %. By, however, providing the same catalyst composition in all reaction compartments it is also possible to perform (kinetic) measurements at higher conversion amounts (> 15 %) and other process parameters can be measured, such as the stability of the catalyst in a reaction over a prolonged period.

In a further embodiment, the at least two reaction compartments are equipped to accommodate at least one support for a second material, for example molybdenum wafers, with the catalyst material thereon. This can, for example, be achieved by means of slots arranged in the longitudinal direction of each reaction compartment. By, for example, using thin wafers (thickness of the order of magnitude of 100 pm) with a thin coating of catalyst material on either side thereof, the surface area of catalyst material in a reaction compartment can be as high as possible.

In a further embodiment of the microreactor in question, the outlet section comprises at least one sampling element, for example three, for discharging reaction product to an analytical instrument (for example a mass spectrometer) connected to the outlet section.

Multiple sampling elements, for example in the form of a thin tube, can be used to make the measurements with the analytical instruments more reliable or to measure a possible radial distribution in the reaction compartment.

To stop a reaction of the reactants outside the reaction section in a controlled manner, in a further embodiment the outlet section contains a cooling device.

According to a further embodiment, the reaction section contains a first temperature sensor that is positioned at a start of the reaction section and a second temperature sensor that is positioned at an end of the reaction section. This makes it possible to determine a temperature profile in the axial direction of the microreactor or to check whether the reaction is proceeding under isothermal conditions. The temperature sensors are preferably arranged in a partition between the at least two reaction compartments.

In a further embodiment the reaction section is provided at the periphery thereof with a heating device for heating the reaction section. The microreactor furthermore contains a third temperature sensor that is positioned on the outside of the reaction section. This enables control of the heating device and in combination with data from the second temperature sensor it is possible to obtain a temperature profile in the radial direction.

In a further aspect the present invention relates to an inlet section for feeding reactants to a reactor, for example a microreactor according to the present invention, wherein the inlet section is provided with a multiplicity of slit-defining walls positioned in parallel, wherein the inlet section can be connected to the reactor in such a way that the slit- defining walls positioned in parallel are perpendicular to a largest dimension of cross- sections of reaction compartments of the reactor. This provides a uniform distribution of the reactants between the various reaction compartments and in each reaction compartment.

In a further embodiment the inlet section contains a heating device for heating the reactants. As a result of the shape of the inlet section this is extremely suitable for use as a heating element as well or for providing space for a separate heating element.

To control the heating device in the inlet section, in a further embodiment this section is provided with a fourth temperature sensor. In addition, this sensor also makes it possible, together with the first temperature sensor, to measure a radial temperature profile at the start of the reaction section.

In one embodiment the inlet section is made of molybdenum, for reasons comparable to those discussed above with regard to the choice of molybdenum for (part of) the microreactor. The molybdenum must have a protective coating for protection against corrosion, at least on surfaces that can come into contact with the reactants, to make the inlet section suitable for oxidising or ammoxidising reactants. Protective coatings comparable to those discussed above for the reaction section can be used for this purpose.

Brief description of the drawings The present invention will now be discussed in more detail on the basis of a number of illustrative embodiments, with reference to the appended drawings, in which Fig. 1 shows a cross-sectional view of a first embodiment of a microreactor according to the present invention; Fig. 2 shows a cross-sectional view along the line II-II in Fig. 1 ; Fig. 3 shows a cross-sectional view along the line in-m in Fig. 2 ; and Fig. 4 shows a front view of an inlet section as used in the microreactor in Fig. 1.

Detailed description of illustrative embodiments Microreactors can be used to carry out tests on catalyst-assisted reactions with a high throughput, also referred to as a high throughput experimental reactor (HTER). In industry existing processes are optimised and new product processes developed. In this context finding an optimum catalyst in a specific process is extremely important. Such HTERs are used to be able to test various catalyst compositions rapidly under different process conditions. In particular, the selectivity of different catalysts for reaction products as a function of time under different process conditions can be tested using such HTERs.

For this invention the aim is, in particular, to find a microreactor that is suitable in particular for reactions of a specific type, specifically highly exothermic reactions that take place in an isothermal manner at a high temperature (up to 700 °C or even higher). One example thereof is the ammoxidation of ethane to give acetonitrile (AHR =-506 kJ/mol) with the use of a zeolite (molecular sieve crystal) as catalyst. Such a reaction generates a large amount of heat under essentially isothermal conditions of up to 700 °C or even higher. Known test reactors are not suitable for these conditions.

One embodiment of a microreactor 1 according to the present invention is sketched in cross-section in Fig. 1. The microreactor 1 generally comprises an inlet section 2, a reaction section 3 and an outlet section 4. The microreactor 1 has an essentially cylindrical shape (see Fig. 2 below).

The gases to be reacted are introduced into the microreactor 1 in the inlet section 2 via an inlet tube 15 (with, for example, an external diameter of 3 mm). An inlet housing 16 connects the inlet tube 15 to the reaction section 3 via a diffuser/heating element 17. The diffuser/heating element 17 serves to heat the reaction gases to the reaction temperature and to produce an essentially uniform distribution of flow over all reaction compartments 10.

The function and construction of the diffuser/heating element 17 is described in more detail below with reference to Figs 3 and 4.

The construction of the reaction section 3 is shown in Fig. 2 in a cross-section along the line II-II in Fig. 1. The reaction section comprises an essentially cylindrical housing 5 with a heating device 6 coaxially around it. A multiplicity of reaction compartments 10 (eight in the embodiment shown, but this can, of course, be a greater or smaller number of reaction compartments 10) is arranged in the housing 5. In the embodiment shown, the reaction compartments 10 are divided into two groups, with a central partition 8, which is located on the axis of the housing 5, between them. There is a dividing wall 7 between reaction compartments 10 of each group. Each reaction compartment 10 is provided with slots 11 arranged in the longitudinal direction of the housing 5, in which slots 11 wafers (not shown) provided with catalyst material can be inserted. Therefore, in the microreactor 1 shown eight different catalyst compositions can be tested.

An outlet section 4 is fixed to the reaction section 3 (once again see Fig. 1). The outlet section 4 comprises a number (three shown) of sampling tubes 21 (for example made of stainless steel) per reaction compartment 10, which are able to feed the reaction product further towards an analytical instrument 27, for example via a multi-position valve 28 connected to the sampling tubes 21. The analytical instrument 27 can be, for example, a mass spectrometer. By using several sampling tubes 21 per reaction compartment 10 it is possible to obtain an internal check on the results from the analytical instrument 27. With the aid of the microreactor 1 and the analytical instrument 27 it is possible in this way to measure intrinsic rates of reaction for eight different catalyst compositions. Further details of the outlet section 4 are discussed in more detail below with reference to Fig. 3.

To make the microreactor 1 suitable for exothermic reactions at high temperatures (for example 700 °C or even higher), such as the ammoxidation of ethane to give acetonitrile, the material of the housing 5 is so chosen that this has high resistance to heat and good thermal conductivity. One example of such a material is molybdenum.

Molybdenum has a coefficient of thermal conductivity of 138 W/mK and a melting point of 2890 K. In addition, molybdenum has a modulus of elasticity of 330 109 Pa and a low coefficient of expansion of 5 10-6 K-l, which makes it possible, inter alia, to achieve thin dividing walls 7. To prevent problems caused by different coefficients of expansion and the like, the other elements of the microreactor are preferably made of the same material or interface elements made of, for example, ceramic material are used. In particular, the supports for the catalyst material (the wafers) are also made of molybdenum, so that good discharge of heat from the reaction region to the surroundings remains guaranteed. It is also guaranteed that even at high temperature the reaction compartments 10 and the wafers with catalyst material remain in a well-defined mutual relationship as a result of the identical coefficient of expansion. All of this ensures that the microreactor 1 is suitable for isothermal conditions.

By providing a different catalyst composition on the wafers in each reaction compartment 10 it is possible to test eight catalyst compositions in parallel. In the reaction section 3 the reaction gases are converted into specific products depending on the catalyst composition in a specific reaction compartment 10. In the outlet section 4, the gases are cooled to prevent undesired subsequent reactions and they are distributed over the sampling tubes 21 to a mass spectrometer for analysis. The microreactor 1 in question thus enables a rapid test of eight different catalyst compositions with regard to catalyst activity, product selectivity and the stability of the catalyst as a function of time. All of this is under varying process conditions, such as composition of the reaction gases and the flow rate and temperature thereof.

By putting wafers with the same catalyst composition in each reaction compartment 10, the microreactor 1 in question can also be used for testing at higher conversions (> 10 to 15 %). This makes it possible also to test a specific catalyst composition for (kinetic) process conditions during a prolonged period and at higher conversions.

The wafers with catalyst material can be placed in the reaction section 3 after removal of, for example, the entire inlet section 2. By this means the openings in the reaction compartments 10 provided with slots 11 are exposed, as a result of which the wafers can easily be slid into place.

In a further embodiment, which is suitable in particular for the application of the microreactor 1 for reactions where oxidising elements play a role, the molybdenum material of the housing 5 (more specifically the reaction compartments 10 of the housing 5) is provided with a coating (not shown) providing protection against corrosion to prevent the molybdenum being converted into molybdenum oxides. The protective coating can, for example, be made of molybdenum silicide (MoSi2) or molybdenum boride (MoB), which provides protection at temperatures of up to 700 °C. For protection at even higher temperature (for example 1000 °C), the protective coating can, as an alternative, be, for example, a 10 pm thick coating of hafnium. The hafnium coating can, for example, be applied by electrodeposition from molten halide material (NaCl-KCl-HfCl4, NaCl-KCl- K2HfF6). In a further variant use is made of ALD (atomic layer deposition) to obtain a protective coating of aluminium oxide. This gives protection at temperatures up to approximately 550 °C. The parts of the inlet section 2 of the microreactor 1 are preferably also provided with a protective coating. Because the (molybdenum) wafers are already provided with a layer of catalyst material, it is not necessary to provide these with a protective coating against corrosion.

A cross-section of the microreactor 1 along the line Ici-m in Fig. 2 is given in Fig. 3.

As described above, the inlet section 2 comprises an inlet tube 15 with an external diameter of approximately 3 mm. The inlet housing 16 and the diffuser/heating element 17 are, for example, made of molybdenum. As can be seen from the combination of Figs 3 and 4, the element 17 comprises a large number of slit-defining walls 19 that are parallel to one another in the direction of flow of the microreactor 1. The slit-defining walls 19 ensure heating of the reaction gases flowing in. This can be controlled by a thermocouple 33 arranged in the inlet housing 16 or in the diffuser/heating element, which thermocouple 33 is, for example, coupled to a PID controller for the heating element 17. The slit-defining walls 19 are positioned perpendicularly to the reaction compartments 10, as a result of which the flow resistance ensures uniform distribution of the reaction gas stream in the radial direction and thus in all reaction compartments 10, over a broad range of flow rates (100 to 1000 ml/min). The central section or central partition 8 of the housing 5 is, of course, not accessible to reaction gases. To prevent disruptions in flow by the central section 8 in reaction compartments 10 located alongside one another, the diffuser element 17 is constructed with a corresponding intermediate part 18 and a permanent molybdenum join is made between the central section 8 and the intennediate part 18.

It is pointed out that the inlet section 2 described here can also be used in other (micro) reactors and can be regarded as a separate component. By this means the said advantages with regard to uniform gas flow distribution and simultaneous heating can also be achieved with other reactors.

The reaction section 3 is produced from a one-piece cylinder (diameter 31 mm, length 47 mm) of molybdenum and comprises 8 reaction compartments 10 provided with microstructures and 47 mm long, 10 mm wide and approximately 2 mm high. The reaction compartments 10 are separated from one another by 250 jim thick dividing walls 7. Each reaction compartment can be filled with eight molybdenum wafers (length 40 mm, width 10 mm and thickness 100 p. m) which are provided on each side with a zeolite coating (for example ion-exchanged ZSM-5 or BEA). The slots 11 in each reaction compartment 10 for accommodating the wafers are also 40 mm in length. The final 7 mm of the reaction compartment 10 (indicated by reference numeral 25) provides room for the sampling system of the microreactor 1 (see below). From the standpoint of structural engineering, this can easily be achieved by producing the final 7 mm separately from the remainder of the housing 5 and then joining the parts to one another using gold solder.

Two thermocouples or comparable temperature sensors 31,32 are arranged in the central partition 8, each at a different end of the housing 5. This makes it possible to obtain a temperature profile in the axial direction of the microreactor 1, so that it is possible to monitor the isothermal conditions. The microreactor is furthermore provided with a further thermocouple 34, which has a two-fold purpose. On the one hand, this further thermocouple 34 is used to control the heating device 6 with the aid of, for example, a PID controller. On the other hand, the further thermocouple 34 can be used together with the thermocouple 32 in the partition 8 to determine a radial temperature profile. Analogously, the thermocouples 31 and 33 can be used to obtain a radial temperature profile of the microreactor 1.

The sampling system in the outlet section 4 can also be seen in Fig. 3 (see also Fig.

1). Three sampling tubes 21 (for example with external diameter 1.2 mm and internal diameter 1.0 mm) are used per reaction compartment 10, as a result of which the sampling system comprises a total of 24 sampling tubes 21. The multiple sampling tubes 21 per reaction compartment 10 can be used either for data validation of the results from the analytical instrument 27 or for a possible measurement of radial concentration distributions within a reaction compartment 10. Each tube 21 can be selected via a multi-position valve 28, which is positioned immediately downstream of the outlet section 4, as a result of which the gases produced can be fed to a mass spectrometer 27 for analysis. Residual gases are removed by means of two discharge tubes 21 that are arranged in a cooling section 23.

The sampling tubes 21 are pushed three mm into the relevant reaction compartment 10 in the axial direction of the microreactor 1. This is sufficient to prevent crosstalk between sampling tubes 21 of different reaction compartments 10. Furthermore, in this way a gap of four mm is produced between the end of the wafers with catalyst and the sampling tubes 21, as a result of which cooling of the reaction compartments 10 is prevented. The four mm gap also provides the opportunity for gas diffusion of streams from the various wafers in the reaction compartment 10, as a result of which an average measurement for all wafers can be carried out.

The outlet section 4 also contains a cooling system that consists of a cooling block 23 (for example made of stainless steel) that is separated from the hot reaction section 3 by an insulating ring 24 (for example made of ceramic material). The gas phase reaction downstream of the reaction section 4 can thus be stopped effectively by cooling the temperature of the outlet gases within milliseconds from the reaction temperature to approximately 150 °C, making use of circulating oil through a cooling channel 26 in the cooling block 23.

To prevent leaks (including at high temperatures), the various parts of the microreactor are pressed firmly against one another. To prevent radiation of heat to the surroundings, in a further embodiment the microreactor 1 is furthermore provided with a layer of insulating material. To control the gas stream from the selected sampling tube 21 (for example in order to make this the same as the gas stream around the outside of this sampling tube 21, isokinetic extraction), a flow/pressure regulator 29 can be provided downstream of the multi-position valve 28 (see Fig. 1). The flow/pressure regulator 29 can also form part of the analytical instrument 27. The entire system of microreactor 1, tubing system and multi-position valve 28 is, for example, installed in a ventilated room at a temperature of 150 °C in order to prevent adverse effects as a result of condensation of water vapour on the tube walls.

The present invention has been explained above on the basis of a number of embodiments. It will be clear to those skilled in the art that numerous modifications and variants are possible that fall within the scope of protection as defined by the appended claims.