EP3475463A1

EP3475463A1 - Method of forming a coating

Info

Publication number: EP3475463A1
Application number: EP17730217.1A
Authority: EP
Inventors: Nikolay Cherkasov; Evgeny REBROV; Alex IBHADON
Original assignee: University of Warwick
Current assignee: University of Warwick
Priority date: 2016-06-22
Filing date: 2017-06-20
Publication date: 2019-05-01
Also published as: CN109642321A; WO2017220590A1; US20190161864A1

Abstract

There is provided a method of forming a coating within an internal pathway. The method comprises the following steps: a) Providing a body having an internal surface which defines an internal pathway within the body, the body having an inlet and an outlet both communicating with the internal pathway for passage of a fluid successively into the inlet then through the internal pathway and then out of the outlet; b) Introducing a liquid solution into the internal pathway so as to fill at least a portion of the internal pathway with the liquid solution, the liquid solution comprising a solute capable of undergoing thermal decomposition; c) Heating the liquid solution while the liquid solution fills said at least a portion of the internal pathway to a sufficient temperature so that the solute undergoes thermal decomposition to form a decomposition product within said at least a portion of the internal pathway. The heating step forms a coating comprising the decomposition product on at least a part of the internal surface that borders the internal pathway.

Description

Method of forming a coating

The invention relates to a method of forming a coating within an internal pathway, or within internal spaces of a porous body.

There are many known applications where it is desirable to form a coating within an internal pathway. In one such application, a porous solid coating is formed on an internal surface of a tube so as to make a column that may be used for chromatography. In another such application, a catalytic coating is formed on an internal surface of a channel so that chemical reactions may be performed within the channel. In yet another such application, a catalytic coating may be formed on an internal surface of a porous body so as to allow chemical reactions to be performed within the porous body.

A known type of method for forming a coating within an internal pathway involves deposition of a coating material from a sol containing solid particles of the coating material suspended in a liquid. The liquid is evaporated leaving the solid particles which form the coating. Examples of this are described in an article by Cherkasov, Ibhadon and Rebrov, entitled: Novel synthesis of thick wall coatings of titania supported Bi poisoned Pd catalysts and application in selective hydrogenation of acetylene alcohols in capillary microreactors, Lab Chip, 2015, 15, 1952. In one example in this article, a titanium dioxide sol was introduced into a fused silica capillary tube. The capillary tube was heated to cause evaporation of the liquid leaving a porous coating of titanium dioxide. In another example, Bi-poisoned palladium catalytic nanoparticles were mixed with a titanium dioxide sol and the mixture was introduced into a fused silica capillary tube. The capillary tube was heated to cause evaporation of the liquid leaving a coating comprising a porous solid titanium dioxide and the Bi-poisoned palladium nanoparticles, with the nanoparticles being supported within the porous solid titanium dioxide.

Despite significant advances taught by the article of Cherkasov, Ibhadon and Rebrov, the formation of coatings by deposition from sols suffers from drawbacks. In particular, sols of suitable coating materials, such as metal oxides, are viscous, non-Newtonian fluids that tend to clog narrow capillary tubes. Liquid evaporation tends to be slow requiring long preparation times. It is often difficult to prepare thick coatings (greater than a few μηη) using a single deposition step. It can also be difficult to prepare coatings on long tubes (greater than about lm). In addition sols tend to be unstable over time and so use of freshly made sols is often required.

US 2004/0033319 Al ('319) discloses a method of forming a coating on the inner surface of a tube. The method comprises providing a liquid containing an organic metal compound within the tube. Heat is applied and the organic metal compound decomposes to form the coating. However, unlike the current invention, in which at least a portion of the length of an internal passageway is filled with a liquid solution, '319 teaches that the liquid should be provided in the tube as a thin film coating on the inner surface of the tube and that it is the thin film coating that is subject to decomposition. The approach adopted in '319 can lead to an uneven coating because the thin film of liquid can become unevenly distributed on the inner surface of the tube before decomposition is complete. The '319 document recognizes this problem and attempts to overcome it by partially decomposing the liquid film with either UV irradiation or ozone treatment to increase viscosity of the liquid film before decomposition is complete. Nevertheless, it is still desirable to provide improved coating methods.

US 2015/0147562 Al ('562) discloses a method in which channels and pores of a porous body are coated with a layer of phosphorous-containing alumina. The '562 document also relies on forming the coating from a thin film coating of liquid.

In accordance with a first aspect of the invention, there is provided a method of forming a coating within an internal pathway, comprising: providing a body having an internal surface which defines an internal pathway within the body, the body having an inlet and an outlet both communicating with the internal pathway for passage of a fluid successively into the inlet then through the internal pathway and then out of the outlet; introducing a liquid solution into the internal pathway so as to fill at least a portion of the internal pathway with the liquid solution, the liquid solution comprising a solute capable of undergoing thermal decomposition; heating the liquid solution while the liquid solution fills said at least a portion of the internal pathway to a sufficient temperature so that the solute undergoes thermal decomposition to form a decomposition product within said at least a portion of the internal pathway; the heating forming a coating comprising the decomposition product on at least a part of the internal surface wherein said at least a part of the internal surface borders the internal pathway.

The use of a liquid solution rather than a sol may overcome or ameliorate the problems associated with the viscosity and unstable nature of sols. Faster formation of the coating may also be possible.

In preferred embodiments, the current invention provides a coating that has a relatively good homogeneity in terms of thickness.

In many instances, the current invention can be used to prepare relatively thick coatings (e.g. greater than 5 μιη) in a single heating step. In addition, the current invention may allow formation of a coating extending along a relatively long internal pathway (e.g. greater than lm in length).

Preferably, the internal pathway is a channel which has first and second openings at the inlet and outlet respectively and which is fully enclosed by the internal surface between the first and second openings. For example, the body may be a tube and the internal pathway may be an internal channel or lumen of the tube. By way of another example, the body is a cartridge.

In another preferred embodiment, the body has a plurality of channels extending parallel to one another and between the inlet and the outlet. Each one of the channels has a first opening at the inlet and a second opening at the outlet. Each channel may be fully enclosed by a respective internal surface between the first and second openings. Alternatively the parallel channels may be interconnected. The inlet may be a formation which connects the first openings, such as a manifold or chamber. Alternatively, the inlet may simply be the first openings when considered collectively. Likewise the outlet may be a formation which connects the second openings. Alternatively, the outlet may simply be the second openings considered collectively.

Channels may be rectilinear but do not need to be.

In any embodiment which has one or more channels (such as a tube), the or each channel may have any practical cross-sectional shape. For example, the channel(s) may be circular in cross-section. In a body which has a plurality of channels, the channels do not need to have the same cross-sectional shape and/or size as one another. The cross-sectional shape and/or size may vary along the length of a channel.

Each channel will have a maximum cross-sectional dimension. This is simply the largest dimension extending in a straight line across the channel in a cross-sectional direction. Hence, when the channel has a circular cross-section, the maximum cross-sectional dimension will be a diameter. When the channel has a square cross-section, the maximum cross-sectional dimension will be a diagonal extending from one corner of the square to the opposite corner. If the cross-section of the channel is constant along the length of the channel, then the maximum cross-sectional dimension will be present all along the length of the channel. Alternatively, if the channel cross-section varies along the length of the channel, then the maximum cross-sectional dimension may only occur at a certain point or points along the length of the channel. Preferably, the or each channel has a maximum cross-sectional dimension of less than 10 mm. In this case, the maximum cross-sectional dimension may be less than 5 mm, less than 4 mm, less than 3 mm, less than 2 mm, less than 1 mm, less than 0.5 mm, less than 0.4 mm, less than 0.3 mm, less than 0.2 mm or less than 0.1 mm.

A particularly preferred body is a tube which has a channel. The channel has a cross-section which is circular and of constant diameter along the length of the tube.

In an alternative preferred embodiment, the body is porous and the internal pathway is formed by a plurality of interconnecting internal spaces within the body. In this case, the inlet may be a plurality of external openings of the porous body. Alternatively, the inlet may be a formation, such as a chamber, which connects a plurality of external openings of the porous body. The outlet may be a plurality of external openings of the porous body (but different openings from those of the inlet). Alternatively, the outlet may be a formation, such as a chamber, which connects a plurality of external openings of the porous body.

Preferably, the internal pathway has a length of at least 0.1m. In this case, the length may be at least 0.2m, at least 0.5m, at least 1m, at least 5m, or at least 10m. In many cases, the internal pathway will be elongate having a length extending between the inlet and the outlet that is significantly longer than the maximum cross-sectional dimension. One example of this is when the body is a tube. In such cases, a potential problem is that, during the heating stage, gas bubbles may form within the liquid solution and expel liquid solution (i.e. in liquid form) from the internal pathway. The expulsion of liquid solution (in liquid form) is undesirable, although expulsion of gas formed from the liquid solution during heating is generally inevitable and acceptable. For example, if the body is a tube which is filled with a column of the liquid solution and the tube is heated uniformly in an oven, gas bubbles may form in the column of the liquid solution causing expulsion of liquid solution. The following preferred embodiment avoids or minimises the expulsion of liquid solution.

In this preferred embodiment, the internal pathway is elongate having a length extending between the inlet and the outlet. The portion of the internal pathway that is filled with the liquid solution is filled with an elongate body of the liquid solution, the elongate body of liquid solution having first and second ends. The heating is performed while one of the inlet and the outlet is closed and the other one of the inlet and the outlet is open. The first end of the elongate body of the liquid solution lies closer than the second end to the open one of the inlet and outlet along the length of the internal pathway, and the second end of the elongate body of the liquid solution lies closer than the first end to the closed one of the inlet and outlet along the length of the internal pathway. The heating comprises applying heat progressively to successive regions of the elongate body of the liquid solution starting at the first end of the elongate body of the liquid solution and moving towards the second end of the elongate body of the liquid solution. In this way, expulsion of liquid solution from the liquid pathway is reduced or prevented.

For example, when the internal pathway is the channel of a tube and the liquid solution fills the channel to form a column of liquid solution, the outlet of the tube may be blocked and the inlet is left open. The column of liquid solution has a first end near the inlet and a second end near the outlet. The heating is then commenced at the first end of the column of the liquid solution. Only a small length of the column of the liquid solution undergoes significant heating at any one moment in time. The heated liquid solution forms a coating as discussed above and this is generally accompanied by evaporation of a solvent of the liquid solution. The gas formed during the evaporation escapes harmlessly through the open inlet of the tube. Because heating is commenced at the first end of the column of liquid solution, at or near the open inlet of the tube, the danger of expelling liquid solution in liquid form is avoided or reduced. This is because the formation of gas bubbles within the column of liquid solution and spaced from the end of the column is avoided or minimised because only solvent at the end of the column is evaporated. The application of heat progresses towards the closed outlet. This is done sufficiently slowly so that only solvent at the end of the column of liquid solution evaporates. In this way, the formation of the coating and the generation of gas from evaporation of the solvent, progress along the column of liquid solution.

The inlet or outlet may be closed in any convenient manner, such as being blocked in a reversible way by laboratory film, or by a valve.

The rate at which the application of heat moves progressively along the elongate body of liquid solution may preferably be from ΙΟμηι to 10cm per second, and more preferably from 0.1mm to 10mm per second, while being sufficiently slow to prevent or reduce expulsion of the liquid solution.

Preferably, when the body is a tube with the inlet at one end of the tube and the outlet at the other end of the tube, the localised heat is applied by a heat source, and the tube and the heat source are moved relative to one another to cause the progressive application of heat. In this case, the heat source may be annular and extend around the circumference of the tube so that heat is applied simultaneously all around the circumference of the tube.

This method of applying heat progressively is not essential. Instead, the expulsion of liquid solution may be avoided if the heating is performed at a low temperature just sufficient to form the coating. In this case, the heat may be applied uniformly to the body, for example in an oven. Slower preparation times may result from this alternative method.

Preferably, in cases where only a portion of the internal pathway is filled with the liquid solution, the portion that is filled forms at least 1% of the length of the internal pathway, and preferably at least 10%, or at least 20%, or at least 30%, or at least 40%, or at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90% of the length of the internal pathway. Of course, the liquid solution may fill the internal pathway completely. According to a second aspect of the invention, there is provided a method of forming a coating within a porous body, comprising: providing a porous body having an internal surface which defines a plurality of interconnected internal spaces within the porous body; introducing a liquid solution into the internal spaces so as to fill the internal spaces with the liquid solution, the liquid solution comprising a solute capable of undergoing thermal decomposition; heating the liquid solution while the liquid solution fills the internal spaces to a sufficient temperature so that the solute undergoes thermal decomposition to form a decomposition product within the internal spaces; the heating forming a coating comprising the decomposition product on at least a part of the internal surface.

The following preferred embodiments refer to any aspect of the invention.

In one preferred embodiment, the decomposition product is a porous solid. For example, the porous solid may be a porous metal oxide, or a porous carbon decomposition product.

The porous solid may be the only component of the coating. In this case, the body with the coating on the internal surface may be used, for example, for chromatography. In particular, the method may be used to form a coating consisting of a porous solid and covering the internal surface of a tube, such as a capillary tube, so as to form a chromatography column.

Alternatively, the coating may comprise a porous solid decomposition product, such as a porous metal oxide, and one or more additional components. One preferred additional component is a catalyst, such as catalytic metal particles. The porous solid decomposition product may act as a support for the additional component(s). For example, an additional component, such as a catalyst, may be entrapped within the porous solid decomposition product. Where the coating comprises a porous solid decomposition product and an additional component, the additional component may itself be a decomposition product of a further solute in the liquid solution.

Where it is desired to form a coating comprising a porous solid metal oxide decomposition product, the solute which decomposes to form the metal oxide preferably comprises a metal cation and an anion (or another ligand). For example, the solute may be a dissolved metal salt. The metal cation may be, for example, titanium, zinc, aluminium, magnesium, calcium, scandium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, gallium, germanium, strontium, yttrium, niobium, molybdenum, cadmium, indium, tin, antimony, tellurium, barium, tantalum, tungsten, thallium, lead, bismuth, zirconium, and lanthanum or actinium group metals. The anion or the ligand may be, for example, nitrate, acetate, acetyl acetonate, nitrite, chloride, citrate, ammonia, carbonyl, cyclopentadienyl and its derivatives, and anions of organic acids including amino acids. Salts consisting of one such cation and one such anion decompose to form metal oxides at temperatures which are convenient to achieve and which are sufficiently low to avoid damage to many types of body to be coated.

Of the cations mentioned above, titanium, aluminium, magnesium, scandium, yttrium, zirconium, iron, cobalt, nickel, manganese, antimony and tin tend to produce metal oxides which have a greater degree of porosity. Zinc, calcium, vanadium, chromium, copper, gallium, germanium, strontium, niobium, molybdenum, cadmium, indium, tellurium, barium, tantalum, tungsten, thallium, lead, bismuth, and lanthanum or actinium group metals tend to produce metal oxides which have a lower degree of porosity.

Both higher and lower porosity metal oxides may be useful, although for many applications higher porosity metal oxides may be preferred.

For some applications metal oxides which have substantially no porosity may be desired.

Another type of porous solid decomposition product will be referred to as porous carbon decomposition product. Generally, a porous carbon decomposition product comprises at least 80% carbon which is not in compound form, when considered by weight as a percentage of the total weight of the carbon decomposition product. Preferably, the porous carbon decomposition product comprises an amount of carbon which is not in compound form, which is at least 90%, or at least 95%, or at least 96%, or at least 97%, or at least 98% or at least 99%, by weight as a percentage of the total weight of the porous carbon decomposition product. The remainder, if any, of the porous carbon decomposition product may be, for example, carbon containing compounds or other impurities. Suitable solutes which decompose to form a porous carbon decomposition product are organic carbon containing compounds. Carbohydrates (and particularly sugars) are one example of organic carbon containing compounds which undergo thermal decomposition to form porous carbon decomposition compounds. Particularly suitable carbohydrates include glucose, fructose, galatose, ribose, maltose, sucrose, lactose. Derivatives of sugars may also be suitable. One suitable sugar derivative is sorbitol.

As discussed above, the coating may comprise a porous solid decomposition product and a catalyst, the porous solid decomposition product being formed by thermal decomposition of the solute. In this case, to generate the catalyst component of the coating, the liquid solution may also comprise a further solute comprising a further solute metallic cation and a further solute anion (or a further solute ligand). The further solute metallic cation may be selected from the group consisting of: platinum, palladium, rhodium, osmium, iridium, ruthenium, copper, silver, cobalt, iron, nickel or gold. The further solute undergoes thermal decomposition to form solid catalytic metal particles, during the same heating step used to convert the first-mentioned solute to the porous solid decomposition product.

In another embodiment, the liquid solution comprises a first solute and a second solute. The second solute comprises a second solute metallic cation and a second solute anion (or a second solute ligand). The second solute metallic cation is selected from the group consisting of: platinum, palladium, rhodium, osmium, iridium, ruthenium, copper, silver, cobalt, iron, nickel or gold. During the heating step, the first solute undergoes thermal decomposition to form a porous solid, as discussed above. The second solute undergoes thermal decomposition to an oxide of the metal of the second solute metallic cation. Hence, the heating forms a coating comprising the porous solid derived from the first solute and a metal oxide derived from the second solute. Subsequently, the coating can be modified by converting the metal oxide derived from the second solute to solid catalytic metal particles by reduction of the metal oxide derived from the second solute with a suitable reducing agent.

In yet another embodiment, a coating comprising a porous solid is formed as described above, and the coating is then modified by introducing catalytic metal particles into the pores of the porous solid in a subsequent step after the formation of the coating. For example, the subsequent step may comprise introducing a further liquid solution into the internal pathway, or into the internal spaces of the porous body. In this case, the further liquid solution comprises a solute capable of being converted into catalytic metal particles. The conversion could be, for example, by way of thermal decomposition to metal particles, or by way of thermal composition to an oxide followed by reduction of the oxide to metal particles by a suitable reducing agent. Alternatively, the subsequent step may comprise introducing a sol containing the solid metal catalytic particles into the internal pathway, or into the internal spaces of the porous body, so that the solid metal catalytic particles enter into and become entrapped within the pores of the porous solid decomposition product.

In cases where the coating comprises a porous solid decomposition product, the liquid solution may comprise a molecule in solution which acts to increase the mean pore diameter in the porous solid decomposition product. The molecule may be a large molecule which may be decomposed or oxidised during the heating step to leave a relatively large pore in the porous solid. Alternatively, the molecule may be a large molecule which can be washed or evaporated away after the formation of the coating to leave a relatively large pore. The molecule may be a polymer. One type of polymer which is particularly useful for increasing the mean pore diameter is a block copolymer having the structure poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol). Block copolymers of this type are sold under the trade name Pluronic (trade mark) by BASF. Pluronic F127 (trade mark) has been found to be particularly useful for increasing the mean pore diameter. Other polymers that may be used to increase the mean pore diameter are: latex; poly(methyl methacrylate); polystyrene; and cross-linked polymers. One particularly suitable cross-linked polymer is polystyrene-divinylbenzene. Other large combustible molecules may be used.

Where a coating or a modified coating comprises catalytic metal particles, the catalytic metal particles preferably contain at least 80% by weight of the metal not in the form of a compound, and more preferably at least 90%, at least 95%, at least 96%, at least 97%, at least 98% or at least 99% by weight of the metal not in compound form, as a percentage of the total weight of the metal particles.

The coating does not need to comprise a porous solid. For example, the coating may be a metallic coating, which may have catalytic properties. In one such example, the solute comprises a metallic cation and an anion (or another ligand). The metallic cation is selected from the group consisting of: platinum, palladium, rhodium, osmium, iridium, ruthenium, copper, silver, cobalt, iron, nickel or gold. The decomposition product is metal in non- compound form which forms the metal coating. For solutes which comprise a metallic cation selected from the group consisting of: platinum, palladium, rhodium, osmium, iridium, ruthenium, copper, silver, cobalt, iron, nickel or gold, suitable anions include acetate, nitrate, nitrite, citrate, and chloride. Other suitable ligands are carbonyl, ammonia and acetyl acetonate.

The concentration of the solute (or solutes) in the liquid solution can vary greatly. Generally, the greater the concentration, the thicker the coating that is formed. Suitable concentrations may be in the range from 0.1 wt/wt% to 80 wt/wt%.

As discussed above, the liquid solution has one or more solutes dissolved therein. Preferably, the liquid solution does not have any suspended solid therein.

Preferably, in cases where only a part of the internal surface is covered by the coating, the coating covers at least 50% of the internal surface, and preferably at least 60%, or at least 70%, or at least 80%, or at least 90% of the internal surface. More preferably, the part of the internal surface that is covered by the coating is a continuous part. Of course, the coating may cover the internal surface completely.

Preferably, the coating that is formed by the heating step has a thickness of at least 1 μηι, and more preferably at least 2 μιη, or at least 5 μιη, or at least 10 μιη, or at least 15 μηι or at least 20 μπι, in the direction of the cross-section of the internal pathway. Given that the coating is formed in a single heating step, the coating will often be homogenous throughout its thickness without having any discernible layers.

Preferably the liquid solution comprises a solvent selected from the group consisting of: water, methanol, ethanol, toluene, xylene, isopropanol, hexane, tetrahydrofuran, dimethylformamide, acetonitrile, dimethyl sulfoxide, and mixtures thereof. However, other suitable solvents may be used. In many cases, solvents with relatively low boiling points are preferred because they evaporate more quickly during the heating step and this may allow a shorter overall preparation time. The solvent should be chosen such that the solute or solutes of the liquid solution are sufficiently soluble in the solvent. In addition, the solvent should have a boiling point that allows evaporation at a sufficient rate at a temperature that is sufficiently low to avoid damage to the body. In addition, the solvent should preferably not damage the coating or dissolve the coating to any significant extent. Preferably, the body is formed from a material selected from the group consisting of: silica, steel, titanium, copper, aluminium, and plastics. However, other suitable materials may be used. The material should be stable at the temperature used for the heating step. The body may be formed form a plurality of materials.

As discussed above, during the heating step, the liquid solution is heated to a temperature sufficient to cause the thermal decomposition of the solute (or solutes). For example, the temperature may be equivalent to the thermal decomposition temperature of the solute plus a margin. The margin may be 50°C to 100°C. Alternative, the margin may be, for example, up to 500°C. The temperature should also be sufficient to cause evaporation of the solvent of the liquid solution at an acceptable rate, but not too high to damage the material, or materials, of the body.

The method may be used in many applications. For example, the method may be used to provide a tube or cartridge or other body with a coating so as to form a chromatography column. In this case, it may be sufficient for the coating to consist of a porous solid without any additional components. Alternatively, the method may be used to coat an internal surface with a catalytic coating so that chemical reactions catalysed by the coating may be performed in an internal pathway, or within the internal spaces of a porous body. This may be particularly useful for coating internal channels of micro-reactors or mili-reactors.

The following is a more detailed description of embodiments of the invention, by way of example, with reference to the appended drawings, in which:

Figure 1 is a schematic representation of a heating apparatus for applying heat progressively along a capillary tube;

Figure 2 is a cross-sectional scanning electron microscope image showing a porous coating on an internal surface of a capillary tube;

Figure 3 is a graphical representation of a coating thickness distribution along the axis of the capillary tube shown in Figure 2; and

Figure 4 is a graphical representation of a pore diameter distribution of the porous coating shown in Figure 2. Referring to Figure 1, the heating apparatus comprises an annular heating element 10, a stepper motor 11, and a pair or rollers 12, 13. The rollers 12, 13 are mounted on the stepper motor 11 and spaced from one another so that a capillary tube 14 can be placed between the rollers 12, 13 and gripped by the rollers 12, 13. The rollers 12, 13 are driven by the stepper motor 11 so that the rollers 12, 13 can displace the capillary tube 14, when so gripped, in an upward direction as shown in Figure 1.

The annular heating element 10 has an internal opening 15 having a diameter large enough to receive the capillary tube 14 so that the heating element 10 lies close to the external surface of the capillary tube 14. The rollers 12, 13 and the heating element 10 are positioned so that the capillary tube 14 is received in the opening 15 of the heating element 10 while the capillary tube is gripped by the rollers 12, 13. As shown in Figure 1, the heating element 10 extends around the external circumference of the capillary tube 14. The heating element 10 is capable of applying heat to the capillary tube 14 all around the circumference of the capillary tube 14.

Example 1

The following is an example of the use of the method to form a coating on the internal surface of a capillary tube. The heating apparatus shown in Figure 1 is used to perform the heating step of the method.

A fused silica capillary tube (10 m long, 0.53mm i.d.) was filled with a liquid solution of zinc (ii) nitrate hexahydrate (6.0 g, Sigma-Aldrich, 98%), palladium (ii) acetate (0.130 g, Sigma- Aldrich, 98%), Pluronic F127 (0.4 g, Sigma-Aldrich) in methanol (15 mL, Fischer Scientific, 99.9%).

The capillary tube 14 was then closed at one end with a shut off valve (not shown) and the other end was left open. The capillary tube 14 was placed between the rollers 12, 13 of the heating apparatus shown in Figure 1 with the open end of the capillary tube 14 located upwards and with the open end lying within the internal opening 15 of the annular heating element 10. The temperature of the annular heating element was set to 300°C.

The capillary tube 14 was then moved upwardly through the internal opening 15 of the heating element 10 at a constant displacement speed of 3mm s^"1. During this process, the volume of the liquid solution located within the annular heating element 10 at any point in time was heated and formed a coating on the internal surface of the capillary tube 14. The methanol solvent within the heated volume evaporated and was expelled as a gas from the open end of the tube. As the capillary tube was displaced upwardly, the application of heat moved along the capillary tube 14 progressively towards the lower closed end of the capillary tube 14 so that the formation of the coating and the evaporation of the methanol solvent also progressed towards the lower closed end of the capillary tube 14.

The capillary tube 14 was then washed with methanol (100 μί min ¹ for 60 min) and dried at 110 °C.

After the washing step, the capillary tube 14 had a continuous coating which covered the entire internal surface of the capillary tube 14. The mass of the coating obtained was 6.5 mg m ¹ and the palladium loading was 3.4 % by mass as a percentage of the total mass of the coating. The coating consisted of zinc oxide formed by thermal composition from the zinc nitrate together with palladium particles formed by thermal decomposition from the palladium acetate.

Figure 2 shows a representative scanning electron microscopy (SEM) cross-sectional image of the capillary tube 14 with the coating consisting of plate-like crystallites of ZnO. As determined by X-ray diffraction (results not shown), the coating comprised hexagonal zincite crystallites 25±4 nm in diameter. Transmission electron microscopy showed the palladium nanoparticles to be 3-5 nm in diameter

Studies on the axial distribution of the coating thickness (see Figure 3) showed that the coating was uniform with an average thickness of 3.6 μηη and standard deviation of 2.4 μιτι. The maximum coating thickness observed was less than 10 μιη, so no narrow places in the capillary tube 14 were present where high pressure drop and pore diffusion limitations might otherwise have been expected. These data agree with very low pressure drop (of 0.2 bar) in the system when the coated capillary tube was used for a hydrogenation reaction as described below in Example 2. Nitrogen physisorption studies (see Figure 4) showed that the coating was mesoporous with an average BJH desorption pore diameter of 26 nm and BET specific surface area of

16 m²g

Example 2

Solvent-free hydrogenation of 2-methyl-3-butyn-2-ol (MBY) was performed using the coated capillary tube 14 prepared in Example 1. Briefly, hydrogen and MBY from a mass-flow controller and a syringe pump, respectively, were combined in a T-joint and passed through the coated capillary tube 14 which was placed in a temperature-controlled water bath. The flow rate of hydrogen was constant, 19 mL min ¹ (STP), while the reaction temperature and the MBY flow rate were varied to optimise the 2-methyl-3-buten-2-ol (MBE) yield.

For every reaction temperature, MBE yield achieved a maximum at a certain MBY flow rate, where the product of MBY conversion and the MBE selectivity was the highest. At a lower MBY flow rate, the residence time increased leading to over-hydrogenation to 2-methyl-2- butanol (MBA); while at a higher MBY flow rate the decreased residence time lead to lower MBY conversion. The hydrogenation rate increased with temperature and resulted in the shift of the maximum MBE yield to higher flow rates. Regardless of the reaction temperature, the maximum MBE yield was above 95% and MBE selectivity was higher than 98% up to 90% MBY conversion. High selectivity of Pd/ZnO catalysts was caused by the in situ formation of an PdZn alloy resulting in the decreased adsorption of alkene species on the catalyst surface.

Example 3

The following is an example of the use of the method to form a coating within a porous body.

First a porous body was prepared as follows. A stainless steel tube having an external diameter of 6.35mm and an internal diameter of 4.2mm was filled with porous cylinders formed from silicon carbide (SiC) foam. Each cylinder had an external diameter of slightly less that the internal diameter of the tube, so as to be a close fit within the tube, and a height of about 5mm. In total, enough silicon carbide foam cylinders were used, end-to-end within the tube, to fill a 20cm length of the stainless steel tube. In this way, a porous body comprising the stainless steel tube and the silicon carbide foam cylinders is formed. Each silicon carbide foam cylinder has an internal surface which forms a plurality of interconnecting internal spaces, and the interconnecting internal spaces form an internal pathway through the foam cylinder from one end the foam cylinder to the other end.

The porous body was then flushed by passing petroleum ether and acetone down the filled tube while subjecting the porous body to ultrasonic treatment. This flushing step serves to remove surface contaminants.

The tube was then filled with an aqueous solution of 5 wt% zinc nitrate hexahydrate and closed at one end with a valve. The aqueous solution filled the internal spaces of the silicon carbide foam cylinders.

The tube, filled with the zinc nitrate solution, was then displaced, open end first, at a rate of O.lmm/s, into a vertical tube furnace held at a temperature of 350°C. Once the tube had been introduced completely in the furnace it was left within the furnace for a further 30 minutes. The application of heat caused the zinc nitrate to undergo thermal decomposition to form a zinc oxide coating. The water was evaporated and expelled harmlessly from the open end of the tube as gas.

The porous body was then flushed with water to remove loosely bound material and dried at 120°C in an oven for 2 hours.

After drying, the internal spaces of the silicon carbide foam cylinders were coated with zinc oxide. The mass of the coating obtained was 70mg for the 20cm length filled tube.

Claims

1. A method of forming a coating within an internal pathway, comprising: providing a body having an internal surface which defines an internal pathway within the body, the body having an inlet and an outlet both communicating with the internal pathway for passage of a fluid successively into the inlet then through the internal pathway and then out of the outlet; introducing a liquid solution into the internal pathway so as to fill at least a portion of the internal pathway with the liquid solution, the liquid solution comprising a solute capable of undergoing thermal decomposition; heating the liquid solution while the liquid solution fills said at least a portion of the internal pathway to a sufficient temperature so that the solute undergoes thermal decomposition to form a decomposition product within said at least a portion of the internal pathway; the heating forming a coating comprising the decomposition product on at least a part of the internal surface wherein said at least a part of the internal surface borders the internal pathway.

2. A method according to claim 1, wherein the internal pathway is a channel which has first and second openings at the inlet and outlet respectively and which is fully enclosed by the internal surface between the first and second openings.

3. A method according to claim 2, wherein the body is a tube.

4. A method according to claim 1 or claim 2, wherein the body has a plurality of channels extending parallel to one another and between the inlet and the outlet, each one of the channels having a first opening at the inlet and a second opening at the outlet.

5. A method according to any one of claims 2 to 4, wherein the or each channel has a maximum cross-sectional dimension of less than 10 mm, and preferably less than 5 mm, 4 mm, 3 mm, 2 mm, 1 mm, 0.5 mm. 0.4 mm, 0.3 mm, 0.2 mm or 0.1 mm.

6. A method according to claim 1, wherein the body is porous and the internal pathway is formed by a plurality of interconnecting internal spaces within the body.

7. A method according to claim 6, wherein the body comprises a tube and at least one porous member within the tube, wherein the porous member provides the internal surface and the plurality of interconnecting internal spaces forming the internal pathway.

8. A method according to any preceding claim, wherein the internal pathway has a length of at least 0.1m, preferably at least 0.5m, more preferably at least lm, and most preferably at least 10m.

9. A method according to any preceding claim, wherein the internal pathway is elongate having a length extending between the inlet and the outlet, wherein said at least a portion of the internal pathway that is filled with the liquid solution is filled with an elongate body of the liquid solution, the elongate body of liquid solution having first and second ends, wherein said heating is performed while one of the inlet and the outlet is closed and the other one of the inlet and the outlet is open, the first end of the elongate body of the liquid solution lying closer than the second end to the open one of the inlet and outlet along the length of the internal pathway and the second end of the elongate body of the liquid solution lying closer than the first end to the closed one of the inlet and outlet along the length of the internal pathway, wherein said heating comprises applying heat progressively to successive regions of the elongate body of the liquid solution starting at the first end of the elongate body of the liquid solution and moving towards the second end of the elongate body of the liquid solution, whereby to reduce or prevent expulsion of liquid solution from the liquid pathway.

10. A method according to claim 9, wherein the body is a tube with the inlet at one end of the tube and the outlet at the other end of the tube, wherein said heat is applied by a heat source, and wherein the tube and the heat source are moved relative to one another to cause said progressive application of heat.

11. A method according to claim 10, wherein the heat source is annular and extends around the circumference of the tube and said heat is applied simultaneously all around the circumference of the tube.

12. A method according to any preceding claim, wherein said at least a portion of the internal pathway forms at least 1% of the length of the internal pathway, and preferably at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, or 90% of the length of the internal pathway.

13. A method according to any preceding claim, wherein the coating covers at least 50% of the internal surface, and preferably at least 60%, or at least 70%, or at least 80%, or at least 90% of the internal surface.

14. A method according to any preceding claim, wherein the coating has a thickness of at least 1 μιη, and preferably at least 2 μιτι, 5 μηη, 10 μπι, 15 μιη or 20 μπι, in the direction of the cross-section of the internal pathway.

15. A method of forming a coating within a porous body, comprising: providing a porous body having an internal surface which defines a plurality of interconnected internal spaces within the porous body; introducing a liquid solution into the internal spaces so as to fill the internal spaces with the liquid solution, the liquid solution comprising a solute capable of undergoing thermal decomposition; heating the liquid solution while the liquid solution fills the internal spaces to a sufficient temperature so that the solute undergoes thermal decomposition to form a decomposition product within the internal spaces; the heating forming a coating comprising the decomposition product on at least a part of the internal surface.

16. A method according to any preceding claim, wherein the decomposition product is a porous solid.

17. A method according to claim 16, wherein the liquid solution comprises a molecule in solution which acts to increase the mean pore diameter in the porous solid decomposition product.

18. A method according to claim 17, wherein the molecule is a polymer.

19. A method according to claim 18, wherein the molecule is selected from the group consisting of: a block copolymer based on ethylene oxide and propylene oxide; latex; poly(methyl methacrylate); polystyrene; and cross-linked polymers.

20. A method according to any one of claims 16 to 19, wherein the coating comprises the porous solid decomposition product and a catalyst supported by the porous solid decomposition product.

21. A method according to claim 20, wherein the catalyst comprises metal particles, the metal particles being entrapped within the porous solid decomposition product.

22. A method according to any one of claims 1 to 21, wherein the solute comprises a metallic cation, and wherein the decomposition product is a metal oxide.

23. A method according to claim 22, wherein the metallic cation is selected from the group consisting of: titanium, zinc, aluminium, magnesium, calcium, scandium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, gallium, germanium, strontium, yttrium, niobium, molybdenum, ruthenium, rhodium, palladium, silver, cadmium, indium, tin, antimony, tellurium, barium, tantalum, tungsten, rhenium, osmium, iridium, platinum, gold, mercury, thallium, lead, bismuth, zirconium, and lanthanum or actinium group metals.

24. A method according to claim 23, wherein the metal oxide is a porous solid and the metallic cation is selected from the group consisting of: titanium, zinc, aluminium, magnesium, calcium, scandium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, gallium, germanium, strontium, yttrium, niobium, molybdenum, cadmium, indium, tin, antimony, tellurium, barium, tantalum, tungsten, thallium, lead, bismuth, zirconium, and lanthanum or actinium group metals.

25. A method according to any one of claims 22 to 24, wherein the solute comprises an anion or a ligand, and wherein the anion or the ligand is selected from the group consisting of: nitrate, acetate, acetyl acetonate, nitrite, chloride, citrate, ammonia, carbonyl, cyclopentadienyl and its derivatives, and anions of organic acids including amino acids.

26. A method according to any one of claims 1 to 21, wherein the solute comprises an organic carbon containing compound and the decomposition product is a porous solid which is at least 80% by weight carbon in non-compound form.

27. A method according to claim 26, wherein the decomposition product is at least 90% by weight carbon in non-compound form, and preferably at least 95%, 96%, 97%, 98% or 99% by weight carbon in non-compound form.

28. A method according to claim 26 or claim 27, wherein the solute comprises a carbohydrate.

29. A method according to claim 28, wherein the carbohydrate is selected from the group consisting of: glucose, fructose, galactose, ribose, maltose, sucrose, lactose and their derivatives.

30. A method according to any one of claims 22 to 29, wherein the liquid solution comprises a further solute comprising a further solute metallic cation, the further solute metallic cation being selected from the group consisting of: platinum, palladium, rhodium, osmium, iridium, ruthenium, copper, silver, cobalt, iron, nickel or gold, and wherein said method comprises converting the further solute to form metal particles.

31. A method according to claim 30, wherein the metal particles contain at least 80% by weight of metal that is not in compound form, and preferably at least 90%, 95%, 96%, 97%, 98% or 99% by weight of the metal not in compound form, as a percentage of the total weight of the metal particles.

32. A method according to claim 30 or claim 31, wherein after said conversion the metal particles are entrapped within the decomposition product of the said solute.

33. A method according to any one of claims 30 to 32, wherein the further solute is converted to the metal particles by thermal decomposition during said heating.

34. A method according to any one of claims 30 to 32, wherein the further solute is converted to a metal oxide by thermal decomposition during said heating and then converted to said metal particles by reduction.

35. A method according to any one of claims 1 to 15, wherein the solute comprises a metallic cation, wherein the metallic cation is selected from the group consisting of: platinum, palladium, rhodium, osmium, iridium, ruthenium, silver, rhenium, mercury or gold, and wherein said decomposition product is metal in non-compound form.

36. A method according to claim 35, wherein at least 90% by weight of said coating is formed by said metal in non-compound form, and preferably at last 95%, 96%, 97%, 98%, or 99% of said coating is formed by said metal in non-compound form.

37. A method according to claim 35 or claim 36, wherein the solute comprises an anion or a ligand, and wherein the anion or the ligand is selected from the group consisting of: acetate, nitrate, nitrite, citrate, chloride, carbonyl, acetyl acetonate, and ammonia.

38. A method according to any one of claims 16 to 19, wherein the coating consists substantially of said porous solid decomposition product, and wherein the method comprises the further step, performed subsequently to the formation of said coating, of providing metal particles within pores of said porous solid decomposition product.

39. A method according to any preceding claim, wherein the liquid solution comprises a solvent selected from the group consisting of: water, methanol, ethanol, toluene, xylene, isopropanol, hexane, tetrahydrofuran, dimethylformamide, acetonitrile, dimethyl sulfoxide and mixtures thereof.

40. A method according to any preceding claim, wherein the body is formed from a material selected from the group consisting of: silica, steel, titanium, copper, aluminium and plastics.

41. A method according to claim 40, wherein the body is formed from a material selected from the group consisting of: polyimide, polyamide, FEP, PTFE, and PEEK.