PROCESS FOR PRODUCING NARROW-PORE CATALYST SUPPORTS

Abstract: A process for the preparation of narrow-pore alumina supports having surface areas above 300 m²/g, at least 80% of the pore diameters less than 5 nm, a crush strength greater than 88 N and containing an amount of phosphorus in the range from 0.1% to 4.5% by weight, which process comprises: a) precipitating an acid aluminium salt in an aqueous solution in the presence of a phosphorus-containing compound by contact with a basic aluminium compound, b) aging the precipitate at a temperature in the range between 20°C and 90°C for at least 15 minutes at a pH in the range between 9.0 to 11.0, c) washing the precipitate, d) drying the precipitate, and e) calcining the precipitate at a temperature ranging from 300°C to 900°C.

Description: This invention relates to a process for preparing porous alumina supports having high surface areas and a high proportion of their pore volume in pores having diameters less than 5 nanometres. In the catalytic processing of petroleum feed stocks, it is often desirable to alter the pore structure of the catalyst support in order to accommodate different types of feeds. For example, when processing feed stocks of high metals content, wide-pore supports are used to prevent plugging of the pores by deposition of metals. On the other hand, when processing feed stocks containing no metals or with a low metals content, it may be both technically and economically desirable to use a narrow-pore catalyst support. For purposes of denitrification, narrow-pore catalysts have higher activities and longer lives than catalysts containing large pores which tend to deactivate quickly. The present invention relates to a process for the preparation of narrow-pore alumina supports having surface areas above 300 m²/g, at least 80%, of the pore diameters less than about 5 nm, a crush strength greater than 88 N and containing an amount of phosphorus in the range from 0.1% to 4.5% by weight, which process comprises a) precipitating an acid aluminium salt in an aqueous solution in the presence of a phosphorus-containing compound by contact with a basic aluminium compound, b) aging the precipitate at a temperature ranging from 20°C to 90°C for at least 15 minutes at a pH ranging from 9.0 to 11.0, c) washing the precipitate, d) drying the precipitate, and e) calcining the precipitate at a temperature ranging from 300°C to 900°C. It has been found that by using phosphorus in the precipitation step, a suitable control over the pore size distribution of the support can be obtained. In the process of the instant invention, an acidic aluminium species and a basic aluminium species are utilized in combination with a phosphorus-containing compound to produce a phosphated alumina hydrogel which upon calcination has at least 80%, preferably at least 90%, of its pore volume in pores of less than 5 nm, a surface area greater than 300 m²/g and a nitrogen pore volume of about 0.35 cm³/g to about 0.65 cm³/g. Precipitation of the alumina hydrogels must be carried out by a concurrent titration procedure in order to create a well-defined and constant environment for the precipitation. If the precipitation is carried out sequentially, where an acid species is added to a basic species, or the reverse, where a basic species is added to an acid species, the pH is constantly changing an the support obtained does not have the same pore size distribution as that obtained when the concurrent titration procedure is used. In the instant invention, the hydrogel is prepared by the simultaneous addition of an aqueous alkali metal aluminate and an aqueous solution of an acidic aluminium salt, at least one of which contains a phosphorus-containing compound dissolved therein, to cause precipitation of the alumina gel. Aluminium sulphate, nitrate or chloride are suitable examples of acid aluminium salts with aluminium chloride being preferred. Alkali metal aluminates are used in the basic precipitation solution, with sodium or potassium aluminate being preferred. The phosphorus-containing compound is preferably phosphoric acid and is preferably mixed with the acidic aluminium species prior to precipitation. Alternatively, the phosphorus-containing compound may be sodium or ammonium phosphate and mixed with the basic aluminium species prior to precipitation without significantly affecting the results. In a preferred embodiment, the phosphorus-containing compound is prepared using commercially available 85% phosphoric acid although other material may be utilized. The amount of phosphorus-containing compound added to the acid aluminium species or the basic aluminium species is such that the amount of phosphorous present in the final calcined alumina support is from 0.1% to 4.5% by weight, preferably from 0.5% to 4% and most preferably from 1% to 3%. The presence of greater amounts of phosphorus in the final calcined product results in unacceptably low densities and crush strengths. The temperature and pH of the precipitation are important variables in the preparation of the instant aluminas needed to produce desirable physical qualities and a narrow pore size distribution. Changes in precipitation temperatures and pHs result in changes in porosities. Depending upon the results desired, one skilled in the art can determine the required precipitation temperature and pH within the after-described limitations with minimal experimentation. In the instant invention, a precipitation temperature typically ranges from 20°C to 90°C, and preferably from 50°C to 85°C, and a precipitation pH typically ranges between 5.5 and 10.0, preferably between 5.5 and 8.0, and more preferably between 6.0 and 7.5. The length of time required for the precipitation step is not critical. However, the maximum rate of addition of the acid aluminium species and the basic aluminium species is fixed by the efficiency with which the two streams can be mixed and the pH and temperature of the system effectively controlled. After the precipitation step is completed, the pH of the slurry is adjusted by the addition of the basic aluminate solution to fall in the range from 9.0 to 11.0, preferably 9.5 to 10.5, and aged at a temperature in the range from 20°C to 90°C, preferably 50°C to 85°C for at least 15 minutes. An upper limit on the length of time for aging is not critical and is normally determined by economical considerations. Aging times will typically range from 0.1 to 10 hours, preferably from 0.25 to 5 hours, and more pre ferably from 0.25 to 1 hour. In general, aluminas with acceptable properties are produced by holding the aging temperature equal to the precipitation temperature. The aging pH, however, is critical. Products with acceptable pore size distributions are prepared only when the aging pH is between 9.0 and 11.0, and preferably between 9.5 and 10.5. Compounds prepared at lower pH values contain primarily macropores. The products formed containing primarily macropores have unacceptably low crush strengths. Aging at excessively high pHs will result in materials having excessively high salt contents. After aging, the slurry is washed and filtered in routine fashion to remove substantially all of the salts formed during the precipitation of the hydrogel. The preferred solvent for washing is water although other solvents such as lower alkanols may be utilized. After filtering, the material may be dried, mulled with addition of water, extruded or pelleted and calcined, or alternatively, partially dried, extruded or pelleted, dried more completely and calcined. Drying is accomplished by conventional means. It may be carried out by forced draft drying, vacuum drying, air drying or similar means. Drying temperatures are not critical and depend upon the particular means utilized for drying. Drying temperatures will typically range from 50°C to 150°C. Normally, the material is extruded after drying to the proper loss on ignition (LOI). However, to facilitate extrusion, organic binders and/or lubricants may be added prior to extrusion. After drying, the material is calcined to cause conversion to γ-alumina. The material may be calcined in any atmosphere, reducing, oxidizing or neutral, although air is preferred. However, if binders and/or lubricants are used, the material is heated in an oxygen-containing atmosphere, preferably air, in order to burn out the binders and lubricants. Calcining temperatures will typically range from 300°C to 900°C. Burn-out temperatures will depend on the concentration of oxygen in the burn-out atmosphere as well as the burn-out time involved. Typically, burn-out temperatures will range from 300°C to 900°C. Drying, calcining and burn-out may be combined in one or two steps. Most frequently, the calcining and burn-out steps are combined using an oxygen-containing atmosphere. Certain other processing steps may be incorporated into the above described procedure without deviating from the scope and intent of this invention. For example, prior to the complete drying of the hydrogel, it may be extruded and then dried more completely, followed by calcination. The porous aluminas of the instant invention can be used as catalyst supports or as catalysts per se. As catalysts, the porous aluminas are useful in reactions which can be typically catalyzed by aluminas. For example, aluminas are particularly useful in hydrocracking and isomerization reactions. The porous aluminas of the instant invention are particularly useful when utilized as supports for catalytically active Group VIA metals (Cr, Mo, W) and Group VIII metals. These catalyzed materials can be suitably applied to hydrocarbon conversion processes such as, for example, hydrotreating and hydrogenation. Example 1 Seven hundred and forty grams of reagent grade sodium aluminate were added to 1000 grams of water which was then heated to 60°C in order to effect solution of the materials. Five hundred and forty-two grams of aluminium chloride hexahydrate were added to 760 grams of water. One hundred and twelve grams of 85% phosphoric acid along with 112 grams of water were added to the aluminium chloride solution which was then heated to 60°C. Both solutions were cooled to slightly less than 60°C and placed in dropping funnels. Five thousand grams of water were added to a 10 litre stain less steel bucket equipped with a pH meter, thermometer and stirrer, to serve as the heel. After the pH of the heel was adjusted to 7.0 using the sodium aluminate solution, the two solutions in the dropping funnels were added simultaneously to the well-stirred bucket, maintaining a precipitation pH of 7. When approximately 200 millilitres of the sodium aluminate solution remained, addition of the aluminium chloride solution was stopped and only the sodium aluminate solution was added to adjust the pH of the solution to 10. The solution was aged for one hour at 60°C. The resulting material was filtered and washed in two large Buchner funnels with about 50 litres of water. The excess water from the wet filter cake was removed by vacuum. The hydrogel was then extruded using a small, hand-held extruder, dried overnight at 120°C and calcined in air at 565°C. The properties of the support are listed in Table I. Comparative Experiment A A support was prepared according to Example I, except that the material was aged at a pH of 8. Results of the experiment are listed in Table I. Comparative Experiment B A support was prepared according to Example I, except that no phosphorus-containing compound was added. The results are listed in Table I. a) Measured using an Orion 231 pH meter and Orion electrodes. b) 209 cm³ volume fully settled in a graduated cup and weighed. c) Defined as expressed as percent. d) BET, by nitrogen adsorption/desorption, Micromeritics Digisorb 2500 Instrument. e) By nitrogen adsorption, Micromeritics Digisorb 2500 Instrument. f) Flat plate, single pellet, extrudate approximately 5 mm in length. g) Weight percent determined by neutron activation analysis. h) Determined by mercury intrusion, to 4285 kg/cm² using a Micromeritics Autopore 9200, using a 130° contact angle and 0.473 N/m surface tension of mercury. Numbers listed are percent pore volume except the average pore diameter which is in nanometres.

Claims: 1. A process for the preparation of narrow-pore alumina supports having surface areas above 300 m²/g, at least 80% of the pore diameters less than 5 nm, a crush strength greater than 88 N and containing an amount of phosphorus in the range from 0.1% to 4.5% by weight, which process comprises: a) precipitating an acid aluminium salt in an aqueous solution in the presence of a phosphorus-containing compound by contact with a basic aluminium compound, b) aging the precipitate at a temperature in the range between 20°C and 90°C for at least 15 minutes at pH in the range between 9.0 to 11.0, c) washing the precipitate, d) drying the precipitate, and e) calcining the precipitate at a temperature ranging from 300°C to 900°C. 2. A process according to claim 1, wherein phosphoric acid is present in the acid aluminium salt solution prior to precipitation. 3. A process according to claim 1, wherein sodium- or ammonium phosphate is present in the basic aluminium solution prior to precipitation. 4. A process according to any one of the claims 1-3 wherein the amount of phosphorus in the final calcined support is in the range from 1% to 3% by weight. 5. A process according to any one of the claims 1-4 wherein the precipitation is carried out at a pH in the range between 5.5 and 10.0. 6. A process according to claim 5 wherein the precipitation is carried out at a pH in the range between 5.5 and 8.0. 7. A process according to any one of the claims 1-6 wherein the precipitation is carried out at a temperature between 20°C and 90°C. 8. A process according to claim 7 wherein the precipitation is carried out at a temperature between 50°C and 85°C. 9. A process according to any one of the claims 1-8 wherein the aging pH is in the range between 9.5 and 10.5. 10. A process according to any one of the claims 1-9, wherein the basic aluminium compound is an alkali metal aluminate. 11. A process according to any one of the claims 1-10, wherein the acid aluminium salt is aluminium sulphate, aluminium nitrate or aluminium chloride.

Inventors: KEMP, RICHARD ALAN
Applicants: SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1986
Main classification: B01J35
IPC codes: C10G45, B01J35, B01J21, B01J37, B01J27, B01J32