PROCESS FOR PRODUCING A HIGH SURFACE AREA ALUMINA

Abstract: An alumina having a very high surface area is produced by treating aluminum salt particles with an aqueous solution of ammonium hydroxide and ammonium carbonate at a pH of 9 to 12 to convert the aluminum salt to an ammonium aluminum hydroxycarbonate, then heating the ammonium aluminum hydrocarbonate to between 150<o> and 400<o>C for a period of at least 2 hours to cause its decomposition to alumina.

Description: PROCESS FOR PRODUCING A HIGH SURFACE AREA ALUMINA 05 FIELD OF THE INVENTION This invention relates to a process for produc¬ ing a high surface area alumina that is useful in alumina- base catalysts. 10 BACKGROUND OF THE INVENTION Catalysts are employed in many chemical indus¬ tries to facilitate chemical reactions which would other¬ wise require unduly stringent reaction conditions. Many catalysts are based on alumina, used either alone (such as ,,. in Claus conversion of hydrogen sulfide, alcohol dehydra¬ tion, and olefin isomerization), as a support (such as the Pt/Pd catalysts used in automobile exhaust systems and in gasoline reforming), and as an interactive support (parti¬ cularly in hydrorefining catalysts for petroleum dis- _ n tillates and residual oils). Typical hydrorefining catalysts comprise a Group VIII metal (typically nickel or cobalt) and a Group VIB metal (typically molybdenum or tungsten) sup¬ ported on an alumina base. The preparation of such a 2 _ catalyst may be exemplified by U.S. Patent No. 4,113,661 to Tamm, the disclosure of which is incorporated herein by reference. The alumina present in such catalysts is typically γ-alumina, though some catalysts prepared by higher-temperature calcination contain δ- or θ-alumina. 30 The catalysts are typically particulate, with sizes rang¬ ing from 0.8 mm diameter cylinders to 5-10 mm diameter cylinders, and more complex shapes such as ovals, tri- lobes, rings, etc. being common. Since the effectiveness of an alumina-base cata¬ 35 lyst is dependent in part on the surface area of that catalyst, it would be desirable to have an alumina that has a high surface area so that it could be used in such a catalyst. 40 SUMMARY OF THE INVENTION This invention provides a process for the pro- duction of such a high surface area alumina. Specifically, this invention provides a process which comprises: (a) treating aluminum salt particles with an aqueous solution of ammonium hydroxide and ammonium carbonate at a pH of 9 to 12 to cause a reaction converting at least a substantial proportion of the aluminum salt to an ammonium aluminum hydroxycarbonate; and (b) heating the ammonium aluminum hydroxycarbonate to between 150° and 400°C for a period of at least 2 hours to cause its decomposition to an alumina that has a very high surface area. DETAILED DESCRIPTION OF THE INVENTION The process of this invention is applicable to alumina-base catalysts. In this context, the term "alumina-base catalyst" means a catalyst the base (i.e., the non-catalytic support, unless the support itself is catalytic) of which comprises alumina. Generally, the term refers to a catalyst base comprising at least 50%, usually at least 80%, and preferably at least 90% by weiqht of alumina. The non-alumina components may be, for example, silica or other refractory oxides, aluminosili- cates such as clays or zeolites, etc. To produce an alumina useful in such catalysts, aluminum salt particles are treated with an aqueous solu- tion of ammonium hydroxide and ammonium carbonate to cause a reaction converting at least a substantial proportion of the aluminum salt to an ammonium aluminum hydroxycarbo¬ nate, which is then heated slowly to between 150° and 400, <> C for a period of at least 2 hours to cause its decom- position to said alumina. The alumina should have a BET surface area of at least 300 m /g, preferably at least 450 m * Vg, more prefer- ably at least 800 m /g. It is often desirable to have a catalyst of very high surface area with corresponding small pores—less than 50 Angstroms. Such catalysts would be used in processes not limited by diffusion control or size exclusion of the reacting molecule. In these cases, reaction rates would be directly proportional to surface area. In the first step, aluminum salt particles are treated with an aqueous solution of ammonium hydroxide and ammonium carbonate to cause a reaction converting at least a substantial proportion of the aluminum salt to an ammo¬ nium aluminum hydroxycarbonate. The aluminum salt is preferably either aluminum sulfate, aluminum chloride, or aluminum nitrate. More preferably, the aluminum salt is aluminum sulfate, because of its low cost. However when aluminum sulfate is used, the resulting alumina should be washed with water at a temperature of at least 50°C to remove a substantial part of the sulfur, ammonia, and other residuals from the alu¬ mina. The reaction pH of the first step is between 9 and 12, preferably between* 10 and 11. A typical solution contains ammonium hydroxide and ammonium carbonate. A typical reaction in this case may be: A1 2 ( S0 4 ) 3 ( S ) + 2NH 4 + ( aq # ) + 40H " ( aq # ) + 2 C0 3 2 - ( aq > ) -> 2NH 4 A1 ( 0H ) 2 C0 3 - H 2 0* ( S ) + 3S0 4 2 " ( aq * non-gelatinous, easily filterable In the second step, the ammonium aluminum hydroxycarbonate is heated slowly to between 150° and 400°C for a period of at least two hours to cause its decomposition to said alumina. Preferably, the heating temperature is about 200°C, for at that temperature, decomposition will occur slowly which is important in obtaining the highest surface area. The heating step should be performed gradually. In order to maximize the resulting surface area, the heating step should occur over a period of at least two hours. In a preferred embodiment, an alumina having a BET surface area of at least 800 irr/g is produced by treating aluminum sulfate particles with an aqueous solution of ammonium hydroxide and ammonium carbonate at a pH of 10 to 11 to cause a reaction converting at least a substantial proportion of the aluminum sulfate to NH 4 Al(OH) 2 C0 3 * H 2 0, heating said NH 4 A1(0H) 2 C0 3 * H 2 0 to about 200°C for a period of at least 2 hours to cause its decomposition to alumina; and washing the alumina with water at a temperature of at least 50°C to remove a substantial part of the sulfur, ammonia, and other residuals from the alumina. EXAMPLES The invention will be further illustrated, without limitation, by the following Examples. Example 1 • An ammonium aluminum hydroxycarbonate was prepared by adding 318 grams of Al 2 (S0 4 ) 3 *16H 2 0 to 1 liter of solution containing 3M NH3 and 1.5 M (NH3) 2 C0 . The mixture was reacted for one hour at 70°C and filtered, then the precipitate was dried in a vacuum oven overnight at 150°C. A similar preparation was made by adding dissolved aluminum sulfate to the same ammonical solution. About 160 grams of each of these precipitates were then decomposed in a muffle oven for about sixteen hours at 250°C. Both preparations showed a BET surface area of 840 m 2 /g. Example 2 A large preparation was made by adding solid aluminum sulfate as in the previous example. In this case, 704 grams of hydroxycarbonate were decomposed at 250°C for eight hours. As in the previous example, the large mass of material resulted in a relatively slow heating rate. The BET surface area was 700 /g. Example 3 This example shows the adverse effect that rapid : heat-up has on surface area. A hydroxycarbonate prepared as in previous examples was decomposed at various tempera¬ tures for three hours. The important variable in this example was the rapid heating rate. Rapid heating resulted from placing only about 8 grams of hydroxycarbon- ' ate into a preheated oven. Table I shows that rapid decomposition even at low temperatures reduced the surface area significantly from the very high values seen previously. TABLE I 2 Temperature, °C Specific Surface Area, m /g 200 259 300 312 400 265 500 315 6 60000 333 700 316 800 205 While the present invention has been described with reference to specific embodiments, this application is intended to cover those changes and substitutions which may be made by those skilled in the art without departing from the spirit and scope of the appended claims.

Claims: HAT IS CLAIMED IS: 1. A process for producing an alumina that has a BET surface area of at least 300 m 2 /g, said process comprising: (a) treating aluminum salt particles with an aqueous solution of ammonium hydroxide and ammonium carbonate at a pH of 9 to 12 to cause a reaction converting at least a substantial proportion of the aluminum salt to an ammonium aluminum hydroxycarbonate; and (b) heatinq said ammonium aluminum hydroxycarbonate to between 150° and 400°C for a period of at least 2 hours to cause its decomposition to said alumina. 2. The process of Claim 1 wherein the surface area is at least 450 m- ~ /q . 3. The process of Claim 2 wherein the surface area is at least 800 m 2 /g. 4. The process of Claim 1 wherein the aluminum salt is selected from the group consisting of aluminum sulfate, aluminum chloride, and aluminum nitrate. 5. The process of Claim 4 wherein the aluminum salt is aluminum sulfate. 6. The process of Claim 5 wherein the alumina is washed with water at a temperature of at least 50°C to remove a substantial part of the sulfur, ammonia, and other residuals from the alumina. 7. The process of Claim 1 wherein the pH of the solution is between 10 and 11. 8. The process of Claim 1 wherein the ammonium aluminum hydroxycarbonate is NH 4 A1(0H) 2CO3Η2O. 9. The process of Claim 1 wherein the heating tern- ' perature is about 200°C. 10. A process for producing an alumina that has a BET surface area of at least 800 m 2 /g, said process comprising: (a) treating aluminum sulfate particles with an aqueous solution of ammonium hydroxide and ammonium carbo¬ nate at a pH of 10 to 11 to cause a reaction converting at least a substantial proportion of the aluminum sulfate to NH 4 A1(0H) 2 C0 3 * H 2 0; (b) heating said NH 4 A1(0H) 2 C0 3 * H 2 0 to between 150° and 300°C for a period of at least 2 hours to cause its decomposition to said alumina; and (c) washing said alumina with water at a temperature of at least 50°C to remove a substantial part of the sul¬ fur, ammonia, and other residuals from the alumina. 11. - The process of Claim 10 wherein the heating temperature is about 200°C.

Inventors: MARCANTONIO PAUL J, MARCANTONIO, PAUL, J.
Applicants: CHEVRON RES, CHEVRON RESEARCH COMPANY
Application year: 1985
Main classification: C01F7
IPC codes: C01F7