GB1565800A

GB1565800A - Preparation of catalyst supports

Info

Publication number: GB1565800A
Application number: GB3320575A
Authority: GB
Original assignee: Imperial Chemical Industries Ltd
Current assignee: Imperial Chemical Industries Ltd
Priority date: 1976-11-03
Filing date: 1976-11-03
Publication date: 1980-04-23

Description

(54) PREPARATION OF CATALYST SUPPORTS (71) We, IMPERIAL CHEMICAL INDUSTRIES LIMITED, Imperial Chemical House, Millbank, London SW1P 3JF, a British Company, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates to improved catalysts.

Most modern heterogenous catalysts are supported, i.e. they consist of catalytically active material or sites on the surface of a less catalytically active or completely inert solid, for example a ceramic oxide. The use of a support often improves the performance of the catalyst over that of the catalytically active material alone. Some of the advantages of supporting a catalytically active material are likely to be as follows, although the presence or absence of these advantages usually varies from case to case: (1) The mechanical strength or the thermal stability of the supported catalyst may exceed that of the active phase alone.

(2) The specific surface area of the supported active material may greatly exceed that of the unsupported material, so that greater efficiency in the catalytic use of the material is possible.

(3) The form of the support particles determines the type of flow of fluids within the reactor and to catalytically active sites on the interior surfaces of the solid particles, and the form of the support particles can be controlled much more readily than the form of most unsupported catalysts. For example, a supported catalyst bed may be operable with lower pressure drop and the mass transfer constraints may be less severe than for a bed of unsupported catalytically active material.

(4) It may be simpler to incorporate promoters or secondary catalytically active centres in a supported catalyst than to optimise the chemical structure of an unsupported active solid.

(5) It is possible that in some circumstances the pore structure of the supporting solid might induce capillary condensation from a gaseous process stream, concentrating reagents at the catalytically active sites.

(6) A supported catalyst may be easier to separate from unconverted reagents and products than is an unsupported catalyst.

Because of the above and other advantages, commercial heterogeneous catalysts are frequently used in a supported form. Nevertheless supported catalysts are not without some disadvantages or difficulties which may increase the cost or decrease the efficiency of heterogeneously catalysed operations. For example, the catalytically active material is usually less than a few per cent by weight of the total catalyst. The catalytic reactor bed therefore contains a high proportion of ineffective weight which has to be carried by the reactor shell and its supports. Also, the need for high mechanical strength in general has precluded use of catalyst supports having high porosity.

High porosity is desirable in pieces of solid catalyst because the porosity represents the fraction of the cross-section of the solid through which reagents and products can diffuse to active sites, i.e. the effective diffusivity of reagents and products is proportional to the porosity. In an idealized catalyst piece consisting of uniform, spherical particles, lowest porosity occurs when the catalyst particles are in so-called close packed array. In close packed array, the catalyst has high mechanical strength since each catalyst particle may be m contact with up to twelve of its neighbours. If the idealized catalyst aggregate is now expanded so that each catalyst particle is in touch with fewer, say only six, of its neighbours porosity increases somewhat, say by about 20%, but at the expense of much of the mechanical strength. In practice, it is unusual for catalyst supports to have a porosity greater than 80%.

Further disadvantages of low porosity supports are that they are likely to have either a low total surface area or a low mean pore diameter which may lead respectively to problems such as inadequate thermal stability of the active material or to mass transfer limitations of reactivity or selectivity. In addition, loading of the active material by impregnation of a low porosity support may be tedious and/or expensive since the amount of active material that can be deposited in a single impregnation is limited by the solubility of the precursor of the active material and the volumes of pores in the solid.

High porosity supports are available in the form of ceramic materials which also have high surface areas. However, these materials tend to have chemically active surfaces which may lead to the easy formation of undesired by-products.

We have now surprisingly found that it is possible to form certain synthetic materials into catalysts which combine relatively high porosity with relatively high mechanical strength and whose catalytic activity can be controlled.

Accordingly, the present invention is a method for the preparation of a catalyst support comprising a fibrous material which method comprises the steps of: (i) reducing, if necessary, the staple length of a fibre selected from ceramic fibres, absbestos, fibrous alumina and fibrous zirconia to a value permitting convenient handling, (ii) mixing the said fibre with a suitable binder comprising a hydroxide slurry gel or a metal nitrate or a solution thereof, (iii) forming the mixture prepared in stage (ii) into pieces suitable for use as a catalyst in a process reaction stream, and (iv) treating the formed pieces so that they become rigid.

The method of the present invention is conveniently applied to ceramic fibres although it is also applicable to other fibres. Synthetic ceramic fibres and their composites have recently been proposed as catalyst supports, for example in flameless heaters and automobile exhaust pollution control applications. These supports are produced as fibre mats which are not rigid and typical staple lengths are 2 to 5 cm. Step (1) of the method of the present invention, therefore, is an optional step to reduce the staple length of the fibre, where this is necessary, to a value which will allow the fibre to be formed into catalyst pellets for use in a process reaction stream. Those skilled in this art will be readily familiar with the shape and size of typical catalyst pellets. Suitably the pellets are cylinders with a diameter and length of 2 to 5 mm.

Fibrous materials for use in the method of this invention are ceramic fibres, asbestos, fibrous alumina and fibrous zirconia. Fibres of high surface area say of BET surface area greater than about 50m2/g, are preferred for use in the method of this invention and one method of preparation of high surface area ceramic fibres is described in U.K. Patent specification No. 1,098,595 and in our U.K. Patent Specifications Nos. 1,360,197 and 1,425,934. However, we do not wish the method of this invention to be limited in its application to any one fibre whether or not of high surface area and whether or not prepared by any of the methods described in the aforementioned patent specifications.

Reduction of the staple length, if necessary, of the fibre used can be achieved by any one of various methods, for example grinding, cutting.

The choice of a binder suitable for use in the method of the invention (step (ii)) will depend to some extent on the nature of the fibre being used, on the chemical and physical history of the fibre, and on the type of final product required. Some oxide materials are conveniently bound by admixture with a metal nitrate or a solution thereof, preferably a solution containing a metal already present in the ceramic material. Alternatively, in some cases a hydroxide slurry gel, for example aluminium hydroxide or silica gel may be used.

The binding can be carried out by suspending the fibrous material in a solution of the metal salt or of a silicate and then adding a precipitant. However, if the fibre is porous it may be preferable to add the precipitant first to the solution and then add the fibre. We have found that in certain cases, it is possible to choose the quantities of fibres, precipitate and liquid so that a mixture is prepared which can be formed into a desired shape of catalyst pellet without prior removal of supernatant liquid. However, it is usually necessary to remove the supernatant liquid prior to stage (iii) of the method of the invention.

The method chosen for forming the mixture of fibre and binder will depend to some extent on the form of catalyst support required and on its chemical composition. Suitable methods include pelleting, extruding, dropping of concentrated slurries into hot oil.

The forming process will usually be easier if the mixture of fibre and binder is plastic.

Methods of treating the formed pieces so that they become rigid (step (iv) of the method of the invention) include drying and/or calcination in an inert or reactive atmosphere for example air. Those skilled in this art will be familiar with the conditions used in these methods and with other suitable methods.

For convenience, the products of the method of the invention are termed consolidates.

They can be envisaged as fibrous mats cemented at points of contact by the binder. Usually there will be two distinguishable pore systems in the fibre mass, viz. large pores bounded by external surfaces of fibres and internal pores, within fibres. In addition, it is believed that in some cases there may be an appreciable pore volume within binder zones. It is believed that access of reagents to the small pores within the fibre and binder is easier in consolidates prepared according to this invention than in conventional catalyst aggregates because of the small diameter of the fibres used in comparison with the separation between access channels in normal catalysts.

For some catalytic purposes, it may be desirable to load active catalytic material preferentially in the internal pores of the fibres. Optionally, therefore, the fibre is loaded with active catalytic material at a stage prior to formation of the consolidate, that is prior to step (iv) of the method of the invention. On the other hand, in some cases it may be preferable to have catalytically active material mainly on the outer surface of the fibres. In such cases, it may be convenient to load the active material after consolidation of the fibre.

A further form of the invention involves the use of fibre of low or no porosity and of low say of BET surface area not greater than about 50m2/g, or no internal surface area. In this form of the invention, it is preferred to use an excess of binder for example an amount contributing from 40 to 80% of the total mass of the consolidate after drying or calcination and to select the binder so that after it has been mixed with the fibrous material it develops an internal surface area, which, preferably, is greater than that of the fibrous component.

Those skilled in this art will readily be able to identify suitable binders i.e. those in which pores will tend to develop upon drying and/or calcining because either (i) part of the binder is lost or evaporates leaving pores, or (ii) the binder undergoes crystalline change to zones of higher density separated by voids. Active material could be incorporated in such a solid either after mixing of the fibre and binder, for example by impregnation, or during preparation of the solid, for example by admixture with the binder.

It is believed that catalysts prepared by the method of this invention will have certain advantages over conventional catalysts. For example, the weight of support required in a given volume of reactor is likely to be lower so that the mechanical structure supporting the catalyst bed may be more cheaply constructed. Mass transfer restrictions of activity and selectivity are likely to be less of a problem because of the high voidage of the catalyst support and the small dimensions of the part of the catalyst which contains low diameter pores. This may allow operation with smaller quantities of active material than would otherwise be the case, thus leading to higher feedstock efficiency and to substantial cost savings for the catalytic operation. In certain cases, when the method of the invention is applied to certain forms of support it may be possible to control the location of the active material, for example largely within small pores or largely within large pores. This may also lead to activity or selectivity improvements.

The method of the invention is illustrated by reference to the following Examples.

Example 1 Samples of fibrous alumina were prepared by reducing the staple length of a fibre mat by various methods as outlined in Table 1. The samples were then packed in air into a cylindrical reactor and the bed voidage calculated from measurements of the bed length and weight of fibre. Table 1 summarises the methods used to reduce the staple length and the results obtained. The results are based on a true fibre density of 4.0 g/ml. and a pore volume of 0.15 ml/g. The calculated bed voidages exclude the fibre pore volumes.

TABLE 1 Fibre treatment to Fibre staple Voidage of reduce staple length untamped bed As received fibre mat 20 to 50 mm Cut into--l cm length 10 mm 0.98 Shredded in a "Mouli" mill 1 to 6 mm 0.98 Ground in pestle and 10 to 200 ijm 0.93 mortar Compacted into disc, ground, recompacted 10 Fm 0.6 and reground These results indicate that the fibrous packing retains its high voidage even when the staple is reduced by a factor of about 103. When the mean particle length was reduced to 1 fibre diameter = 2 to 3ym) the voidage remained higher than values normally experienced with cylindrical pellets having an aspect ratio of about unity (i.e. voidages of about 0.40).

Example 2 A sample of porous fibrous alumina (containing traces of silica) was obtained, 2 - 5cm.

staple. The pore volume was about Q.15 ml/g. and the surface area was about 150 m2/g. as determined by the BET Nitrogen isotherm method. The fibre was reduced to about 2 mm.

staple by passage twice through a "Mouli" mill. 22.3 g. of the fibre was suspended in a solution of 44.5 g. aluminium nitrate hydrate in 250 ml. distilled water. 100 ml. of aqueous ammonium hydroxide (containing 20 ml. of "0.880 ammonia") was now added with vigorous stirring to the slurry. After 30 minutes the slurry was transferred to a sintered grass filter and about 150 ml. of liquor were removed by suction. The filter cake had a doughy consistency and was formed into extrudates by passage through a die of diameter about 1.5 mm. The extrudates were dried for 6 hrs. in an oven at 80 - 100"C and transferred to a furnace in which the temperature was raised 100 C per hour to 500 C:. After 4 hrs at SQQ"C the extrudates were allowed to cool. The measured pore volume and surface area were Q.91 ml/g and 197 m2/g respectively.

WHAT WE CLAIM IS: 1. A method of preparation of a catalyst support comprising a fibrous material which method comprises the steps of: (1) reducing, if necessary, the staple length of a fibre selected from ceramic fibres, asbestos, fibrous alumina and fibrous zirconia to a value permitting convenient handling, (ii) mixing the said fibre with a suitable binder comprising a hydroxide slurry gel or a metal nitrate or a solution thereof, (iii) forming the mixture prepared in stage (ii) into pieces suitable for use as a catalyst in a process reaction stream, and (iv) treating the formed pieces so that they become rigid.

2. A method as claimed in claim 1 in which the staple length of the fibre is reduced by grinding or cutting.

3. A method as claimed in claim 1 or 2 in which the binder is aluminium hydroxide or silica gel.

4. A method as claimed in any one of the preceding claims in which binding is effected by suspending the fibrous material in a solution of a metal salt or of a silicate and then adding a precipitant.

5. A method as claimed in any one of claims 1 to 3 in which binding is effected by adding a precipitant to a solution of a metal salt or of a silicate and then suspending the fibrous material in the mixture of liquid and precipitate.

6. A method as claimed in any one of the preceding claims in which the mixture of fibre and binder is formed into catalyst pieces (stage (iii)) by a method selected from pelleting, extruding and dropping of concentrated slurries into hot oil.

7. A method as claimed in any one of the preceding claim in which the formed catalyst pieces are made rigid (stage (iv)) by drying and/or calcination in an inert or reactive atmosphere.

8. A method as claimed in any one of claims 1 to 6 in which the fibre is loaded with active catalytic material at any stage in the method prior to stage (iv).

9. A method as claimed in any one of claims 1 to 7 in which the rigid catalyst pieces are loaded with active catalytic material.

10. A method as claimed in any one of the preceding claims in which the fibre has a BET surface area greater than 50 m-/g.

11. A method as claimed in any one of claims 1 to 9 in which the fibre has either no internal surface area or a BET surface area of less than or equal to 50 m2/g.

12. A method as claimed in any one of the preceding claim in which the fibre has low or

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. These results indicate that the fibrous packing retains its high voidage even when the staple is reduced by a factor of about 103. When the mean particle length was reduced to 1 fibre diameter = 2 to 3ym) the voidage remained higher than values normally experienced with cylindrical pellets having an aspect ratio of about unity (i.e. voidages of about 0.40). Example 2 A sample of porous fibrous alumina (containing traces of silica) was obtained, 2 - 5cm. staple. The pore volume was about Q.15 ml/g. and the surface area was about 150 m2/g. as determined by the BET Nitrogen isotherm method. The fibre was reduced to about 2 mm. staple by passage twice through a "Mouli" mill. 22.3 g. of the fibre was suspended in a solution of 44.5 g. aluminium nitrate hydrate in 250 ml. distilled water. 100 ml. of aqueous ammonium hydroxide (containing 20 ml. of "0.880 ammonia") was now added with vigorous stirring to the slurry. After 30 minutes the slurry was transferred to a sintered grass filter and about 150 ml. of liquor were removed by suction. The filter cake had a doughy consistency and was formed into extrudates by passage through a die of diameter about 1.5 mm. The extrudates were dried for 6 hrs. in an oven at 80 - 100"C and transferred to a furnace in which the temperature was raised 100 C per hour to 500 C:. After 4 hrs at SQQ"C the extrudates were allowed to cool. The measured pore volume and surface area were Q.91 ml/g and 197 m2/g respectively. WHAT WE CLAIM IS:

1. A method of preparation of a catalyst support comprising a fibrous material which method comprises the steps of: (1) reducing, if necessary, the staple length of a fibre selected from ceramic fibres, asbestos, fibrous alumina and fibrous zirconia to a value permitting convenient handling, (ii) mixing the said fibre with a suitable binder comprising a hydroxide slurry gel or a metal nitrate or a solution thereof, (iii) forming the mixture prepared in stage (ii) into pieces suitable for use as a catalyst in a process reaction stream, and (iv) treating the formed pieces so that they become rigid.

no porosity.

13. A method as claimed in claim 11 or 12 in which binding is effected with an excess of binder.

14. A method as claimed in claim 13 in which the binder contributes from 40 to 80% of the total mass of the product catalytic material after drying or calcination.

15. A method as claimed in claim 13 or 14 in which the binder is one which, after mixing with the fibrous material, develops an internal surface area which is greater than that of the fibrous material.

16. A method of preparation of a catalyst support substantially as hereinbefore described with reference to the Examples.

17. Catalyst supports whenever prepared by a method as claimed in any one of the preceding claims.