Method for producing a thin shell catalyst

Abstract: A catalyst is disclosed comprising a thin outer shell of catalytic material bonded to an inner core of catalytically inert material. The catalyst is made by coating a catalytically inert core such as alpha alumina, with a thin layer of finely divided catalytically active material in a slurry of colloidal boehmite/pseudo boehmite then calcining to convert the boehmite/pseudo boehmite into γ alumina thereby bonding it to the inert core. Catalysts made by this technique containing an outer shell of platinum on fluorided alumina in an inert core of alpha alumina γ are attractive isomerization catalysts.

Description: The present invention relates to a method for producing a catalyst comprising a thin annular shell of catalytically active material on a catalytically inert core. Platinum on fluorided alumina are known to be effective catalysts for the isomerization of waxes and for upgrading distillates and raffinates. The most selective catalysts for the conversion of wax into isomerate oil contains fluoride in the range 0.1 to less than 2 wt.% and employs small size particles of less than 1.59 mm (1/16 inch) diameter. There is an incentive, therefore, to make low fluoride catalyst in particles smaller than 1.59 mm (1/16 inch) diameter but resort to particles of much less than 1.27 mm (1/20 inch), such as 1.27 mm (1/20 inch) trilobes, could result in weaker particles which easily crumble or fracture and thereby contribute to pressure drop and plugging problems in the reactor. Despite these concerns, however, smaller catalytically active particles are desirable as a way of controlling and minimizing the diffusion effect attributable to larger particles. In EP-A-O 075 314 there is disclosure of bifunctional catalysts comprising a thin shell of gamma alumina and nickel oxide as catalyst, coated onto an inactive carrier, which may be alpha alumina. The catalyst coating may be preformed from gamma alumina and a nickel salt. Preformed alpha alumina particles are coated with the catalyst and then calcined. In US-A-4 255 253 there is disclosure of thin shell catalysts in which the catalyst material is preformed from gamma alumina particles impregnated with metal oxide. A coating of the catalyst material is applied to a preformed core of silica, alumina or alumina-silica and the whole is then calcined. According to one aspect of the present invention, there is provided a method of producing a catalyst having a thin annular shell of catalytically active material on catalytically inert core; which method is characterized by : (a) dispersing (i) finely divided, preformed, catalytically active material comprising Group VIB, VIIB, VIII, metal, oxide, sulfide, or mixtures thereof and further comprising at least one activator selected from phosphorus, halogen and boron, in (ii) colloidal boehmite/pseudo boehmite to produce a slurry; (b) providing a thin coating of said slurry on a core which is catalytically inert but which has surface sites capable of the chemical bonding hereafter specified; and (c) calcining the coated core so as (i) to effect conversion of the boehmite/pseudo boehmite into gamma alumina and (ii) to effect during that conversion, chemical bonding between at least some of the forming gamma alumina and surface active sites on the core. Conversion of boehmite/pseudo boehmite into gamma alumina effectively binds the thin shell containing the finely divided catalytically active material or finely divided preformed catalyst, to the inert core of, e.g., alpha or gamma alumina. The thin active shell may contain any desired amount of catalytically active material taking into account that a sufficient amount of boehmite/pseudo boehmite must be available for conversion into gamma alumina to generate the bonding matrix. Because the catalytically active material is present only in the thin shell and not throughout the entire particle, less catalytically active material is needed, based on the total weight of catalyst, versus conventional catalysts with a uniform distribution of active component throughout the particle. In the method according to the present invention, the finely divided preformed catalyst may be mixed with the colloidal boehmite/pseudo boehmite slurry (20% in solvent) in a ratio of about 1:1 to 1:4 by weight, preferably 1:2 finely divided preformed catalyst to boehmite/pseudo boehmite ratio, which when calcined results in finely divided preformed catalyst to gamma alumina ratios of the same range. In general it is preferred that the final catalyst has from 0.1 to 5% by weight catalytically active metal and up to 15% by weight promoter in the outer layer, preferably 0.3 to 2 wt% catalytically active metal and 0.1 to 2 wt% promoter. The thin layer of boehmite/pseudo boehmite- catalytically active material, and, following calcination the resulting thin layer of catalytically active material in gamma alumina, may range from 10 to 500 micrometers (µm) in thickness, preferably 20 to 300 µm in thickness, the thinner shell being preferred for those processes in which diffusion effects of feed/product into the catalyst are detrimental. The colloidal boehmite/pseudo boehmite is produced by mixing boehmite/pseudo boehmite with a solvent such as water, ketone, alcohol, ether, etc, preferably water. The boehmite/pseudo boehmite is preferably present in the solvent at a level of 1 to 30%, more preferably 10 to 20% boehmite in solvent producing colloidal boehmite/pseudo boehmite. When the object is to produce an acid catalyst, such as a catalyst useful for wax isomerization or waxy hydrocarbon upgrading it is necessary that the colloidal boehmite/pseudo boehmite slurry have no basicity attributable to strong bases such as those containing alkaline or alkaline earth cations. Basicity attributable to bases having volatile cations such as ammonium cation can be tolerated because such volatile cation components can be easily driven off during the calcination step which converts the boehmite/pseudo boehmite into gamma alumina and such bases do not have a negative impact on the acidity of the catalyst. In general it is preferred that in producing an acidic catalyst the colloidal boehmite/pseudo boehmite slurry be neutral or acidic (pH 7 or less). The inert core particles are coated with the slurry by any convenient method such as rolling, milling, dipping, spraying, etc. the object being to produce a thin, even coating. The coated particles are dried and calcined. Calcination is conducted under conditions sufficient to convert the boehmite/pseudo boehmite into gamma alumina. Such conditions can include heating the coated particles in air at 500 to 600°C for from 1 to 24 hours. Calcination converts the boehmite/pseudo boehmite into a gamma alumina bonding matrix in which is suspended the catalytically active material, which chemically bonds to the inert core producing a strong, physically stable particle. When producing isomerization catalysts by the process of the invention and employing a preformed isomerization catalyst such as platinum on fluorided alumina, it is preferred that the amounts of platinum and fluorine in the preformed catalyst be such that after mixing with the colloidal boehmite/pseudo boehmite slurry and taking the dilution effect into consideration the amount of platinum and fluorine in the outer shell of the finished catalyst is in the range of 0.1 to 2.0% platinum and 0.1 to 10.0% fluorine. When used for isomerization, the catalyst is usually activated prior to use by heating in a hydrogen atmosphere (e.g., pure or plant hydrogen (i.e., ∼ 60 to 70 vol% H 2 )) to from 350°C to 500°C for from 1 to 48 hours or longer. A typical hydrogen activation profile may be a period of 2 hours to go from room temperature to 100°C with the catalyst being held at 100° from 0 to 2 hours, then the temperature is raised from 100°C to about 350°C to 500°C, preferably 350°C to 450°C over a period of 1 to 50 hours with a hold at the final temperature of from 0 to 24 hours, (preferably 24 hours). Similarly, hydrogen activation can be accomplished by going from room temperature to the final temperature of 350°C to 500°C preferably 350°C to 450°C in 1 to 50 hours. The catalysts made by the method of the present invention are useful for isomerizing wax to liquid products and for upgrading waxy distillates and raffinates. One such wax isomerization process is described and claimed in our EP-B-0 542 528. The wax which is isomerized can be any natural petroleum wax identified as slack wax, recovered by the solvent dewaxing of petroleum hydrocarbon feeds, or synthetic wax such as that obtained by the Fischer-Tropsch process. Natural waxes such as the aforementioned slack wax can contain appreciable amount of oil. It is desirable to deoil the slack wax before the isomerization process. Slack waxes containing anywhere between 0 to 50% oil can be isomerized, although the more desirable oil content is about 35% oil and less. Natural waxes also can contain heteroatom compounds, that is compounds containing nitrogen and sulfur. Such heteroatom compounds are known to deactivate noble metal containing isomerization catalyst. Before isomerizing such heteroatom containing wax feeds, it is necessary to reduce the sulfur and nitrogen content of the feed. These heteroatom containing wax feeds should be hydrotreated to reduce the level of heteroatom compounds to levels commonly accepted in the industry as tolerable for feeds to be exposed to isomerization catalysts. Such levels will typically be a nitrogen content of about 1 to 5 ppm and a sulfur content of about 1 to 20 ppm, preferably 2 ppm or less nitrogen and 5 ppm or less sulfur. The hydrotreating step will employ typical hydrotreating catalysts such as Co/Mo, Ni/Mo, or Ni/Co/Mo on alumina under standard, commercially accepted conditions, e.g., temperature of about 280°C to 400°C, space velocity of about 0.1 to 2.0 V/V/hr, pressure of from about 3.45 to 20.69 MPa (500 to 3,000 psig) H 2 and hydrogen gas rates of from about 89 to 890 std. m 3 /m 3 (500 to 5000 SCF/bbl). As previously stated, synthetic waxes such as those obtained from Fischer-Tropsch synthesis processes can also be used as the wax feed to the isomerization process. Because such waxes are usually free of sulfur and nitrogen compounds, hydrotreating to remove S and N is not needed. Synthetic waxes, however, may contain other polar or oxygenated components and trace metals which may be removed prior to isomerization to improve product daylight and oxidation stability. The waxes are also very high melting and should be softened somewhat to facilitate handling prior to isomerization. These two goals can be accomplished by treating the synthetic wax with a hydrotreating catalyst and hydrogen to reduce the oxygenate and trace metal levels of the wax and to partially hydrocrack/isomerize the wax to lighter and lower melting point materials. This pretreatment of synthetic Fischer-Tropsch waxes is one aspect of the invention taught in U.S. Patent 4,943,672. Isomerization over the catalyst is conducted at a temperature of 300°C-400°C, 3.45 to 20.69 MPa (500 to 3000 psi) H 2 ; 178 to 1780 std m 3 /m 3 (1000-10,000 SCF/bbl) H 2 , and 0.1-10.0 LHSV, preferably 320°C-385°C, 6.9 to 10.34 MPa (1000-1500 psi) H 2 , and 1-2 V/V/hr. An integrated process to produce lube base stock oils or blending stocks by isomerizing waxes is disclosed in U.S. Patent 5,059,299. In the accompanying drawings : Figure 1 compares the activity-selectivity of the thin shell catalyst according to the invention and a conventional hydroisomerization catalyst of comparable platinum and fluorine content for wax isomerization at constant space velocity; and Figure 2 compares the activity and selectivity for wax isomerate of a thin active shell inert core catalyst according to the invention with a traditional uniform particle catalyst of comparable platinum and fluoride loading at three different space velocities. Examples 110 cc of 1.59 mm (1/16 inch) gamma alumina extrudate were air calcined at 1050°C for 3 hours to convert them into catalytically inert alpha alumina extrudates as confirmed by x-ray powder diffraction. 22 grams of preformed active catalysts comprising 0.3% Pt 1.0%F on 1.27 mm (1/20 inch) gamma alumina trilobes made as disclosed in U.S. Patent 4,906,601 was ground into a fine powder and slurried with a two fold excess (44 grams) of NYACOL COLLOIDAL ALUMINA SOL (20% Boehmite in H 2 O). The resulting gray slurry was transferred to teflon sheet and alpha alumina extrudates were rolled through the slurry to yield thin and evenly coated particles. Periodically, the remaining slurry was diluted with deionized water to maintain a workable slurry consistency. Coated particles were dried at room temperature then air calcined at 550°C for 3 hours to convert the boehmite into gamma alumina thereby binding the coating to the alpha alumina core particle. Small scale experiments were performed to determine the fraction of active catalyst. The calcined thin shell catalyst weighed 17.64 grams following treatment of the alpha alumina core particle which weighed 10.37 grams. Thus the thin shell coating weighed 7.27 grams constituting 41.2% of the total catalyst. This thin shell was made up of finely divided preformed catalyst in gamma alumina (from the conversion of boehmite into gamma alumina). Insofar as 22 grams of preformed catalyst was slurried in 44 grams of a 20% colloidal boehmite solution (8.8 g boehmite), the total dry weight of coating is 30.8 grams. The total amount of active preformed catalyst in the total coating slurry is 71.4%. The final amount of active catalyst in the finished catalyst, therefor, constituted 41.2% of 71.4% or 29.4 wt.% of the finished catalyst (Catalyst A). The finished thin shell catalyst contained 0.24% platinum and 0.8% fluorine in the gamma alumina layer which on the basis of the total weight of catalyst represents 0.07 wt.% platinum and only 0.24 wt.% fluorine. Catalyst A comprised an alpha alumina core with a radius of 0.8 mm as determined by scanning electron microscopy. It was determined that the average coating thickness was 0.25 mm based on optical microscopy and from the weight increase. 80 cc of the thin shell catalyst were charged to a lab hydrofining unit run in an upflow isothermal mode at 6.9 MPa (1000 psig) pure H 2 pressure, 445 std m 3 /m 3 (2500 SCF/bbl), 1.0 to 0.5 LHSV and a temperature range of 350 to 390°C. Another run conducted on a 100 cc charge of 0.3% Pt on 0.75% fluoride 1.59 mm (1/16 inch) alumina extrudate (fluorided using NH 4 F)(Catalyst B) in a lab hydroprocessing unit was employed as a basis for comparison of the performance of the present catalyst. Both runs were conducted on hydrofinished 600N slack wax with ∼ 20.5 wt% oil content as feed. This comparison run was conducted at a pressure of 6.9 MPa (1000 psi) H 2 , 1.0 LHSV, gas rate of 890 st m 3 /m 3 (5000 SCF/bbl) and a temperature of 308 to 337°C. Both runs were conducted under temperature conditions selected so as to obtain maximum yield of dewaxed oil using the particular catalyst being used. The results are presented in Table 1. COMPARISON OF NEW AND CONVENTIONAL HYDROISOMERIZATION CATALYSTS (Feed: Hydrofined 600N Slack Wax, 6.9 MPa (1000 psig) Upflow) Operating Unit Catalyst A B Temperature 350 380 380 385 390 311 321 331 337 Gas treat rate (SCFH 2 /bbl) std m 3 /m 3 (2500) (2500) (2500) (2500) (2500) (5000) (5000) (5000) 5000) -445 - -890 - LHSV, v/v/H 1.0 1.0 0.5 1.0 1.0 1.0 1.0 1.0 1.0 Conv. to 370°C-on FD 1.8 9.5 15.2 12.6 14.0 14.7 21.9 34.4 47.9 Oil Content of 370°C+ 28.6 41.8 69.4 56.7 58.7 50.5 60.4 75.5 79.8 DWO Yield on Feed 28.1 47.8 57.9 48.0 48.3 43.0 47.1 49.5 41.5 The performance of catalyst A is excellent even though the effective catalyst was only 29.4% by weight of the conventional catalyst charge in terms of platinum and fluoride, i.e. (0.07 wt% Pt and 0.24 wt% F) vs (0.3% Pt, 0.75 wt% F) in catalyst B. The object in this hydroisomerization process is to maximize the conversion of wax molecules to isomerate oil while minimizing conversion to lower boiling species (e.g., to 370°C-). Catalyst A shows an about 8 wt% dewaxed oil yield advantage. While it required more severe conditions to attain this (0.5 LHSV and 380°C) this was accomplished with 70% less catalyst. Therefore, where space velocity or throughput is not critical, this would be a more selective inexpensive alternative. Figures 1 and 2 again clearly illustrate the advantage. Fig. 1, shows activity and selectivity relationship for catalyst A or catalyst B at one space velocity. While it is seen that even at a reactor temperature of 390°C vs 337°C, the DWO yield of the conventional catalyst could not be exceeded. Catalyst A is much more selective ( ∼ 13 wt% Fuels vs ∼ 47 wt% Fuels). In Fig. 2, the space velocity is reduced to 0.75 and 0.5. At a space velocity of 0.5, catalyst A shows about an 8% DWO yield increase over the optimum of the conventional catalyst but at only ∼ 15% vs ∼ 35% conversion to fuels. Here again, where throughput is not critical, the new thin shell catalyst exceeds the conventional one in terms of selectivity.

Claims: A method for producing a catalyst having a thin annular shell of catalytically active material on a catalytically inert core; which method is characterized by : (a) dispersing (i) finely divided, preformed, catalytically active material comprising Group VIB, VIIB, VIII, metal, oxide, sulfide, or mixtures thereof and further comprising at least one activator selected from phosphorus, halogen and boron, in (ii) colloidal boehmite/pseudo boehmite to produce a slurry; (b) providing a thin coating of said slurry on a core which is catalytically inert but which has surface sites capable of the chemical bonding hereafter specified; and (c) calcining the coated core so as (i) to effect conversion of the boehmite/pseudo boehmite into gamma alumina and (ii) to effect during that conversion, chemical bonding between at least some of the forming gamma alumina and said surface active sites on the core. The method of claim 1, wherein the calcining step comprises heating the coated particles in air at from 500 to 600°C for from 1 to 24 hours. The method of claim 1 or 2, wherein the coating step provides a coating thickness of from 10 to 500 microns. The method of any one of claims 1 to 3, wherein the preformed catalyst employed comprises platinum on fluorided gamma alumina. The method of any one of claims 1 to 4, wherein the core material employed is alpha alumina.

Inventors: CODY IAN ALFRED, HAFEZ MAHMOUD MOSTAFA, ZINKIE DAVID NORMAN, CODY, IAN ALFRED, HAFEZ, MAHMOUD MOSTAFA, ZINKIE,DAVID NORMAN
Applicants: EXXON RESEARCH ENGINEERING CO, EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1992
Main classification: B01J37
IPC codes: B01J23, B01J35, B01J27, B01J21, B01J37, C10G45