Hydrotreating Catalysts Prepared From Hydrogels

This invention relates to a process for preparing highly active alumina hydrogel-derived catalysts.

In the catalytic processing of petroleum feedstocks, it is often desirable to alter the pore structure of the catalyst in order to accommodate different types of feeds. For example, when processing feedstocks of high metals content, the metals tend to deposit rapidly on the catalyst surface and plug the pores of conventional hydroprocessing catalysts, resulting in a loss of catalytic activity for sulfur and nitrogen removal. To facilitate the diffusion of large components into and out of the catalyst and to prevent surface deposits of coke and metals, large pore diameters are required. On the other hand, when processing feedstocks with no metals or with low metals contents, it may be technically and economically desirable to use narrow-pore catalysts. For purposes of denitrification, narrow-pore catalysts have higher activities and longer lives than catalysts containing large pores which tend to deactivate quickly.

Related cases include co-pending applications Ser. No. 924,091, filed Oct. 28, 1986, and Ser. No. 924,092 filed Oct. 28, 1986, which are directed to catalysts prepared by the incorporation of metals solutions into hydrogels.

The present invention relates to a process for the preparation of hydrotreating catalysts containing an element selected from the group consisting of molybdenum, tungsten and mixtures thereof, an element selected from the group consisting of nickel, cobalt and mixtures thereof, and phosphorus incorporated into an alumina hydrogel-derived support. The catalyst is prepared by a process which comprises:

b(a) titrating an aqueous solution of an acid aluminum salt with an aqueous solution of a basic aluminum compound, thereby forming a precipitate,

(b) aging the precipitate at a temperature ranging from about 20.degree. C. to about 90.degree. C. for at least about 15 minutes at a pH ranging from about 11.0 to about 12.5,

(c) washing the precipitate,

(d) mixing the precipitate with a solution containing solubilized salts of a heavy metal selected from the group consisting of molybdenum, tungsten and mixtures thereof, an element selected from the group consisting of nickel, cobalt and mixtures thereof, and a phosphorus-containing compound in an amount from about 0.2 to about 1.5 moles of phosphorus per mole of heavy metal at a pH in the range between about 4.0 and about 8.0 and a temperature in the range between about 25.degree. C. and about 100.degree. C. until adsorption of the metal salts onto the gel is sufficient to yield a final catalyst having from about 1% w to about 5% w nickel and/or cobalt and from about 8% w to about 32% w heavy metal,

(e) extruding the product of step (d), and

(f) drying and calcining the product of step (e) at a temperature ranging from about 300.degree. C. to about 900.degree. C.

It has been found that catalysts prepared from hydrogels aged at high pHs have physical properties better than catalysts prepared from hydrogels aged at lower pHs as well as catalysts prepared by conventional impregnation techniques. Two of the major advantages of the hydrogel route are higher metals utilization and lower costs of catalyst manufacture. The catalysts prepared according to the invention have high surface areas, greater than about 300 m.sup.2 /g; high flat plate crush strengths, greater than about 20 lbs; and substantial portions, greater than about 80%, of their pores with diameters less than about 50 .ANG.. These catalysts are particularly useful for hydrotreating applications.

In the process of the instant invention, a highly active hydrotreating catalyst is prepared by incorporating a metals solution containing an element selected from the group consisting of molybdenum, tungsten and mixtures thereof, an element selected from the group consisting of nickel, cobalt and mixtures thereof, and a phosphorus-containing compound into an alumina hydrogel-derived support prepared by titrating an aqueous solution of an acid aluminum salt with an aqueous solution of a basic aluminum compound.

The alumina hydrogel can be prepared by titrating an aqueous solution of one or more aluminum salt(s) with an appropriate acidic or basic material or solution to cause precipitation of the phosphated alumina gel. One skilled in the art will recognize that the alumina gel can be prepared by titrating an acidic aluminum salt such as, for example, aluminum sulfate, aluminum nitrate or aluminum chloride, in aqueous solution with a basic precipitating medium such as, for example, sodium hydroxide or ammonium hydroxide, or, by titrating an alkali metal aluminate such as, for example, sodium aluminate or potassium aluminate, in aqueous solution with an acidic precipitating medium such as, for example, hydrochloric acid or nitric acid. One skilled in the art will recognize that the adjustment of the pH of an aluminum-containing solution to between about 5.5 and about 10.0 will result in precipitation of the aluminum as aluminum hydroxide or hydrated aluminum oxide.

In a preferred embodiment, the alumina hydrogel is prepared by titrating an aqueous solution of an alkali metal aluminate and an aqueous solution of acid aluminum salt to cause precipitation of the alumina gel. Suitable acidic aluminum salts include aluminum sulfate, aluminum nitrate and aluminum chloride. A preferred species is aluminum chloride. Suitable alkali metal aluminates are sodium aluminate and potassium aluminate. The precipitation can be carried out by adding an aqueous solution of the basic aluminum species to an aqueous solution of the acidic aluminum species or the procedure can be reversed by adding an aqueous solution of the acidic aluminum species to an aqueous solution of the basic aluminum species (referred to as "sequential precipitation"). Preferably, the precipitation in the instant invention is carried out by simultaneously adding the acid aluminum species and the basic aluminum species to cause precipitation of the hydrogel (referred to as "simultaneous precipitation").

The temperature and pH of the precipitation are important variables in the preparation of the aluminas into which metals solutions can be incorporated to form hydrotreating catalysts with desirable physical qualities. Changes in precipitation temperatures and pHs result in changes in porosities. In the instant invention, a precipitation temperature typically ranges from about 20.degree. C. to about 90.degree. C., and preferably from about 50.degree. C. to about 85.degree. C., and a precipitation pH typically ranges between about 5.5 and about 10.0, preferably between about 5.5 and about 8.0, and more preferably between about 6.0 and about 7.5. The length of time required for the precipitation step is not critical. However, the maximum rate of addition of the acid aluminum species and the basic aluminum species is fixed by the rate at which the two streams can be mixed and the pH and temperature of the system can be 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 about 11.0 to about 12.5 preferably about 11.0 to about 12.0, most preferably about 11.5 to about 12.0, and aged at a temperature in the range from about 20.degree. C. to about 90.degree. C., preferably about 50.degree. C. to about 85.degree. C. for at least about 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 about 0.1 to about 10 hours, preferably from about 0.25 to about 5 hours, and more preferably from about 0.25 to about 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 about 11.0 and about 12.5, and preferably between about 11.0 and about 12.0. Compounds prepared at lower pH values contain primarily macropores. The products formed containing macropores have unacceptably low crush strengths. Aging at excessively high pHs will result in dissolution of the alumina gel.

After aging, the slurry is washed and filtered in routine fashion to remove substantially all the salts formed during the precipitation of the hydrogel which can be removed by washing. The preferred solvent for washing is water although other solvents such as lower alkanols may be utilized.

After washing, the hydrogel is reslurried with a solution containing solubilized salts of a heavy metal selected from the group consisting of molybdenum, tungsten and mixtures thereof, and an element selected from the group consisting of nickel, cobalt and mixtures thereof, and a phosphorus-containing compound an amount of from about 0.2 to about 1.5 moles of phosphorus per mole of sufficient to deposit on the final catalyst from about 1% w to about 5% w nickel and/or cobalt and from about 8% w to about 18% w molybdenum or about 10% w to about 32% w tungsten. When mixtures of molybdenum and tungsten are utilized, the final catalyst contains from about 8% w to about 32% w molybdenum and/or tungsten. Alternatively, the solution may contain amounts of nickel and/or cobalt and molybdenum or tungsten in excess of that required to deposit the aforesaid amounts of metals, which excess may be removed by washing following the reslurry step. A typical metals solution can be prepared by combining a molydbenum solution with a nickel and/or cobalt solution. A phosphorus-containing compound is typically added to the nickel and/or cobalt solutions in order to prevent formation of unstable nickel and/or cobalt/molybdenum or nickel and/or cobalt/tungsten complexes when the solutions are combined. As used herein, the term "a phosphorus-containing compound" is generic and refers to one phosphorus-containing compound as well as more than one phosphorus-containing compound. The phosphorus-containing compound is generally selected from the group consisting of phosphoric acid, a phosphate salt or mixtures thereof. Suitable phosphate salts include alkali metal phosphate, alkali metal hydrogen phosphate, ammonium phosphate and ammonium hydrogen phosphate.

The molybdenum solution consists of a water-soluble source of molybdenum such as ammonium heptamolybdate or ammonium dimolybdate dissolved in water. Hydrogen peroxide may also be used to aid in solution preparation in some cases. A preferred method for preparing the molybdenum solution consists of adding hydrogen peroxide to the solution in the range of about 0.1 to about 1.0 mole of hydrogen peroxide per mole of molybdenum. Optionally, a suitable soluble amine compound such as monoethanolamine, propanolamine or ethylenediamine may be added to the molybdenum solution in order to aid in stabilization of the solution.

The tungsten solution typically consists of ammonium metatungstate dissolved in water. A preferred method for preparing the tungsten solution consists of adding hydrogen peroxide to the solution in the range of about 0.1 to about 1.0 mole of hydrogen peroxide per mole of tungsten. In addition, a suitable soluble amine compound such as monoetholamine, propanolamine or ethylenediamine may be added to the tungsten solution in order to aid in stabilization of the solution.

The nickel solution consists of nickel salts dissolved in water and a phosphorus-containing compound. A wide range of nickel compounds are suitable, such as nickel nitrate, nickel acetate, nickel formate, nickel sulfate, nickel oxide, nickel phosphate, nickel carbonate, nickel chloride, and nickel hydroxide. Two compounds that are especially useful are nickel nitrate and nickel carbonate.

The cobalt solution consists of cobalt salts dissolved in water and a phosphorus-containing compound. A wide range of cobalt compounds are suitable, such as cobalt nitrate, cobalt hydroxide, cobalt acetate, cobalt oxalate, or cobalt oxide. The preferred cobalt compound is cobalt nitrate.

The temperature and pH of the reslurry step are both important variables in the preparation of hydrogel-derived catalysts which have acceptable densities and porosities. In general, high temperatures in the reslurry step result in lower density catalysts. The pH of the reslurry step, however, is critical to the formation of catalysts having the desired properties. The reslurry of the hydrogel support with the metals solution is carried out at a pH in the range between about 4.0 and about 8.0, preferably between about 4.0 and about 6.0, adjusting to these pH ranges by adding, if necessary, acid or base to the hydrogel, the metals solution and/or the mixture of the slurry and the metals solution, and a temperature in the range between about 25.degree. C. and about 100.degree. C., preferably between about 25.degree. C. and about 80.degree. C., until adsorption of the metals salts onto the gel is sufficient to yield a final calcined catalyst having from about 1% w to about 5% w nickel and/or cobalt and from 8% w to about 18% w molybdenum or from about 10% w to about 32% w tungsten. Reslurry pHs less than about 4.0 result in dissolution of the alumina gel and reslurry pHs greater than about 8.0 result in failure of the molybdenum or tungsten to adsorb onto the gel. Typically, the times for reslurry will range from about 0.5 to about 2 hours. Optionally, the resulting material can be washed to remove unadsorbed metals and filtered in routine fashion.

Following the reslurry step, the material may be extruded and then dried and calcined; dried, mulled with addition of water, extruded or pelleted and calcined; or 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 about 50.degree. C. to about 150.degree. C.

In a preferred embodiment, the material is extruded and then dried. Alternatively, the material may be extruded after drying to the proper loss on ignition (1.0I). However, to facilitate extrusion, organic binders and/or lubricants may be added prior to extrusion.

After drying, the material is calcined to produce the finished catalyst. 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 typially range from about 300.degree. C. to about 900.degree. 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 about 300.degree. C. to about 900.degree. C. Drying, calcining and burn-out may be combined in one or two steps. Most frequently the calcining and/or 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 catalyst, it may be extruded and then dried more completely, followed by calcination.

The final catalysts are found to have surface areas greater than about 300 m.sup.2 /g, pore volumes ranging from about 0.4 to about 1.2 cc/g and with at least 80% of its pore volume in pores having diameters less than about 50 .ANG.. Crush strengths are greater than about 20 lbs. In general, the metals contents of the final catalysts range from about 1% w to about 5% w, preferably from about 2.5% w to about 4% w nickel and/or cobalt, and from about 8% w to about 18% w, preferably about 10% w to about 14% w molybdenum or about 10% w to about 32% w, preferably about 18% w to about 26% w tungsten.

The catalysts prepared in the instant invention can be suitably applied to hydrocarbon conversion processes such as catalytic cracking, hydrocracking, hydrotreating, isomerization, hydrogenation, disproportionation, dehydrogenation, oligomerization, alkylation, dealkylation and the like.

The catalysts of the instant invention are most commonly employed in hydrotreating feedstocks ranging in volatilities from napthas to flashed distillates. Reaction temperatures will typically range from about 300.degree. F. to about 850.degree. F., preferably from about 500.degree. F. to about 775.degree. F. Reaction pressures are generally within the range of about 200 psig to about 2500 psig, preferably from about 600 psig to about 1500 psig. Reactions are conducted at liquid hourly spaced velocities within the range of about 0.1 to about 10 reciprocal hour.

Multiple uses of these feedstock after treating with the catalysts of this invention are possible. Depending on the particular feedstocks treated, suitable uses can include conversion unit feedstock or finished products such as gasoline, diesel, airline turbine fuel, furnace oils and solvents.

The process for preparing the catalysts of the instant invention will be further described below by the following examples which are provided for illustration and which are not to be construed as limiting the invention.

Seven hundred and forty grams of reagent grade sodium aluminate were added to 1000 grams of water which was then heated to 60.degree. C. in order to effect dissolution of the materials. Five hundred and forty-four grams of aluminum chloride hexahydrate were added to 760 grams of water. Both solutions were cooled to slightly less than 60.degree. C. and placed in dropping funnels.

Five thousand grams of water were added to a 10 liter stainless 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. A total of 1740 grams of sodium aluminate solution and 1216 grams of aluminum chloride solution were used to effect the titration. After these quantities of reagents had been used, the final aging pH of the solution rose to between 11.0 and 12.0. The solution was aged for one hour at 60.degree. C. The resulting material was filtered and washed in two large Buchner funnels with about 50 liters of water. The excess water from the wet filter cake was removed by vacuum. The hydrogel was then divided into two equal portions.

The following solutions were then prepared. 91.71 grams of nickel nitrate as hexahydrate was mixed with 64.59 grams of 85% phosphoric acid and diluted to 1000 milliliters with water. An additional solution of 136.17 grams of ammonium heptamolybdate and 21.21 grams of 30% hydrogen peroxide were diluted to 1000 milliters with water. The two solutions were combined slowly with stirring and one portion of the hydrogel was added and reslurried with one-half of the solution at 80.degree. C. for two hours at a pH of 5.0. At the end of two hours, the slurry was filtered and washed with two liters of water. The excess water from the slurry was again removed by vacuum. The wet gel was then extruded using a small, hand-held extruder using a 0.156" cylindrical die, dried overnight at 120.degree. C. and calcined in air at 510.degree. C. The properties of the catalyst are listed in Tables I and II.

A catalyst was prepared according to Example 1 except that the aging step was carried out at a pH of 10.0. The properties of the catalyst are listed in Tables I and II.

A catalyst was prepared using a conventional dry pore volume impregnation technique. A solution suitable for impregnating a gamma alumina carrier was prepared as follows. A first solution was made by adding 19.39 grams of nickel nitrate and 16.18 grams of phosphoric acid to 5 milliliters of water which solution was then heated to 40.degree. C. and stirred in order to effect dissolution of the materials. A second solution was prepared by mixing 5.5 milliliters of 30% hydrogen peroxide, 30 milliliters of distilled water and 35.20 grams of ammonium heptamolybdate. The mixture was then heated to 40.degree. C. and stirred until all solids were dissolved. When both solutions were at or near ambient temperature, the second solution was slowly added to the first solution while stirring. After cooling and diluting with water to the total pore volume of the support, the solution was added to the gamma alumina support in several small portions with intermediate agitations. The impregnated support was further agitated about 5 minutes, dried for 2 hours at 400.degree.0 F. and calcined in air for 2 hours at 900.degree. F. The properties of the catalyst are listed in Tables I and II.