WO1991013831A1

WO1991013831A1 - Magnesium aluminate with controlled surface area

Info

Publication number: WO1991013831A1
Application number: PCT/GB1991/000315
Authority: WO
Inventors: Ashok Bhattacharya
Original assignee: Rover Group Limited
Priority date: 1990-03-15
Filing date: 1991-02-28
Publication date: 1991-09-19
Also published as: AU7347791A; GB9005805D0

Abstract

A method of preparing magnesium aluminate having a controlled and uniform surface area comprises heating a mixture of magnesium and aluminium salts in an aqueous medium containing a nitrogen compound that decomposes slowly under the conditions of the reaction to release ammonia which reacts with said salts to form a gel comprising magnesium hydroxide and aluminium hydroxide, aging the gel, filtering and washing the aged gel, drying the filtered and washed gel, and calcining the dried gel.

Description

MAGNESIUM ALUMINATE WITH CONTROLLED SURFACE AREA

This invention relates to a method of preparing magnesium aluminate with controlled surface area and to magnesium aluminate prepared by the method. Magnesium aluminate is useful as a catalyst support, but it is important that materials that are used as catalytic supports have a uniform and optimal surface area. For example magnesium aluminate having a very high surface area may be useful in the catalysts disclosed in our co- pending application number PCT/EP89/01231 , whereas magnesium aluminate having a very low surface area may be useful in selective oxidation reactions. Magnesium aluminate having a uniform and very low surface area may also be useful for manufacturing ceramic components.

Magnesium aluminate (MgAl₂04) can display an oxy- spinel structure with close-packed oxygen atoms defining octahedral and tetrahedral sites for metal ions. At normal temperatures the tetrahedral sites are occupied by magnesium ions and the octahedral sites by aluminium ions, but at elevated temperatures some rearrangement can occur resulting in some aluminium ions occupying tetrahedral sites and some magnesium ions occupying octahedral sites. It is possible to replace a proportion of the magnesium ions with other ions, e.g. cerium, zirconium and lanthanide ions. The magnesium aluminate of the present invention may display the oxy-spinel structure, possibly with partial rearrangement of the metal ions as outlined above, and in which some of the magnesium ions, possibly occupying octahedral sites as well as tetrahedral sites, may have been replaced by ions of other elements.

According to one aspect of this invention a method of preparing magnesium aluminate having a controlled and uniform surface area comprises heating a mixture of magnesium and aluminium salts in an aqueous medium containing a nitrogen compound that decomposes slowly under the conditions of the reaction to release ammonia which reacts with said salts to form a gel comprising magnesium hydroxide and aluminium hydroxide, aging the gel, filtering and washing the aged gel, drying the filtered and washed gel, and calcining the dried gel. If the dried gel is calcined at a relatively-low temperature, preferably about 500 to 600° C and up to about 800° C, the resulting product can exhibit a surface area as high as about 250 m² /g. On the other hand, if it is calcined at a higher temperature, e.g. about 1,000 ° C, the resulting magnesium aluminate exhibits a very low surface area e.g. about 0.2 m² /g.

The nitrogen compound is preferably urea. The slow release of ammonia resulting from hydrolysis of the urea is a critical feature of the invention. For example, if a mixture of magnesium and aluminium salts is simply reacted with other ammonium compounds such as ammonium oxalate, the resulting magnesium aluminate end product exhibits only a low surface area. It is thought that the gradual release of ammonia results in slow and uniform precipitation of the hydroxides whereas an excess of ammonia (e.g. resulting from rapid decomposition of an ammonium compound) would lead to rapid and uneven precipitation.

In the first step, the concentration of urea is preferably 15 to 50%. The reaction is preferably carried out under reflux at about 100° C at a pH ranging from 5 to 9. The nature of the magnesium and aluminium salts is not particularly critical, for example nitrates may be used.

The aging step involves allowing the gel to stand, for example for 15 to 24 hours. Aging is preferably carried out at a temperature within the range 18 to 80° C.

The washing step is preferably performed with water. The drying step is preferably carried out over a number of hours, (e.g. 12 hours) at a temperature somewhat in excess of 100° C (e.g. 120° C ).

The calcining step is carried out at a significantly higher temperature (e.g. 550° C ) over a number of hours (e.g. 6 hours) .

Provided that the calcining step is not carried out at a temperature greater than about 800° C, the resulting magnesium aluminate is amorphous. The amorphous product may be used as such as a catalyst carrier and displays a very high surface area, e.g. 150-

250 m² /g. Further heating over a prolonged period and at a higher temperature (over about 800° C and preferably about 1,000° C ) results in the formation of magnesium aluminate having a spinel structure and displaying a very low but uniform surface area which is also suitable for use as a catalyst support or for manufacturing ceramic components.

For example, if the amorphous product obtained by calcining at 550° C is heated at 1,000° C for 24 hours, X-ray diffraction studies have confirmed that it adopts the oxy-spinel structure, and continuation of the heating for a further 60 hours at 1,000° C leads to a product displaying very strong oxy-spinel crystallinity with no residual MgO or A1₂0₃ patterns having a surface area of about 0.2 m /g.

As outlined above, small proportions of other elements may be introduced into the spinel structure to replace a proportion of the magnesium ions. This may be achieved by doping the initial mixture of magnesium and aluminium salts with small quantities of salts of other metals such as zirconium, lanthanum, yttrium, hafnium, scandium, lanthanides such as cerium, and actinides such as thorium. Silicon components such as silicon tetrachloride or silicic acid can also be used. Only a very low proportion of the additional element is required as dopant, e.g. 0.001 to 0.003 moles, as opposed to 0.047 to 0.04 moles of magnesium and 0.9 to 1.1 moles of aluminium, all expressed as oxides.

Addition of small quantities of other metal salts or silicom components to the mixed salt solution prior to the hydrolysis step often leads to a delay in gelation. Thus, delays from 30 minutes to 1 hour have been observed.

The presence of these additional elements tend to stabilise the structure. Thus, the presence of a very small proportion of Zr0₂ leads to a product having a surface area of about 160 m² /g after the calcining step at 500-600° C. Calcining at 1,000°C in this case brings about a much smaller reduction in surface area, i.e. to 60 to 100 m² /g.

The electronic properties of magnesium aluminate may be greatly modified by addition of the small quantities of other elements, as outlined above, enabling the interaction between the support and the catalytic material carried by it to be tailored so that the properties of the supported catalyst can be optimised.

Claims

CLAIMS :

1. A method of preparing magnesium aluminate having a controlled and uniform surface area comprises heating a mixture of magnesium and aluminium salts in an aqueous medium containing a nitrogen compound that decomposes slowly under the conditions of the reaction to release ammonia which reacts with said salts to form a gel comprising magnesium hydroxide and aluminium hydroxide, aging the gel, filtering and washing the aged gel, drying the filtered and washed gel, and calcining the dried gel.

2. A method according to Claim 1 , in which the nitrogen compound is urea.

3. A method according to Claim 2, in which the concentration of urea in the aqueous solution is 15 to 50%.

4. A method according to any preceding claim, in which aging is carried out by allowing the gel to stand for 15 to 24 hours at a temperature from 18 to 80° C.

5. A method according to any preceding claim, in which the calcining step is carried out at a temperature below

800° C, preferably around 500-600° C.

6. A method according to any preceding claim, including the further step of heating the calcined product at a temperature of at least 800° C and preferably around 1,000° C so that it adopts the oxy- spinel structure.

7. A method according to any preceding claim, in which a small proportion of the magnesium ions have been replaced by ions of another element.

8. Process according to Claim 7, in which the other element is selected from zirconium, lanthanum, yttrium, hafnium, scandium, silicon, lanthanides and actinides.