Catalyst preparation

Abstract: A process for the preparation of a metal-containing supported catalyst, having at least one metal or metal compound substantially present in a selected part thereof comprising contacting an aqueous solution of an appropriate metal compound with a reagent capable of changing the valency of said metal during contact with a preshaped support. The resulting catalyst, which may have an egg-shell or an egg-yolk type distribution for the catalytically active component, may be used e.g. in diffusion limited processes as well as in processes wherein the feedstock may contain catalyst poisons, e.g. in residue conversion.

Description: The invention relates to a process for the preparation of metal-containing supported catalysts A problem frequently encountered in catalysed processes is the relatively short life of the catalysts, e.g. in hydrotreating processes especially when employing heavy feedstocks such as residua. In such a process the relatively high concentrations of asphaltenes and/or asphaltic components normally present in such feedstocks tend to result in excessive coke formation and deposition thereof on the catalyst, thereby reducing the active catalyst surface and hence the overall catalytic activity. In recent years a considerable effort has been spent on finding ways and means of extending the catalyst life, especially because of the increasing tendency to use heavier feedstocks. A proposal towards solving the problem of reduced catalyst life as a result of coke and/or metal deposition on the catalyst, has been described in US-A-4378308, and comprises coating the catalyst surface with a hydrated clay mineral, hydrated alumina, hydrated silica or mixtures thereof. It is postulated that such a coating, which generally has a smaller pore size than the original catalyst, protects the catalyst by inhibiting the diffusion of catalyst poisons to the active sites. A thus prepared catalyst can be considered to be a catalyst having a non-uniform distribution of the active component i.e. a catalyst comprising a relatively large core containing the active component, covered by an active component-free outer shell. Although the thus prepared catalysts are likely to be less sensitive to contamination by certain feedstock components or derivatives thereof, they still possess a number of inherent disadvantages. For instance they will suffer from adhesion problems at the interface of the coating and the original catalyst, which will reduce the overall mechanical strength of the coated catalyst. Furthermore the mechanical strength of the coating as such may be inferior to that of the original catalyst and may thus give rise to excessive attrition. By coating an existing catalyst it will be very difficult to maintain the original shape of the catalyst, which may be especially critical in the case of spherically shaped catalysts or catalysts based on carriers having a very specific shape such as trilobes and rings. Finally the actual coating of an existing catalyst introduces an additional preparation step. Hence it will be clear that there is room for improvement in the preparation of catalysts for use in processes wherein catalyst contamination and/or poisoning is likely to occur. In European patent specification 204314 a process is described for providing a catalyst containing an active metal component supported on a porous refractory carrier wherein the metal concentration is higher in the core of a catalyst particle than on the surface. The process comprises treating a refractory carrier with aqueous solution A containing at least one Group Vb, VIb or VIII active metal, taking the treated carrier out of solution A and subsequently treating it with solution B, which contains at least one component selected form the group consisting of anions of the Group VII, ammonium ion, nitrate ion, sulfate ion, hydroxyl ion, phosphate ion, borate ion and organic acids, and finally treating it with solution A again. It is now proposed to prepare metal-containing supported catalysts by a single step process wherein the combination of process conditions and nature of the reactants will determine the ultimate distribution of the metal or metal compound over the support and which results in supported catalysts which do not suffer from the inherent disadvantages of the coated catalysts referred to hereinbefore. The invention provides therefore a process for the preparation of a metal-containing supported catalyst wherein the concentration of at least one metal or metal compound is considerably lower in the outer layer of the catalyst than in the inner portion of the catalyst, comprising contacting an aqueous solution of an appropriate metal compound with a reducing agent or an oxidising agent resulting in a redox reaction either (a) generating hydrogen ions during contact with a basic reacting preshaped support, or (b) generating hydroxyl ions during contact with an acidic reacting preshaped support. It is extremely surprising that in the process according to the present invention it is not only possible to prepare metal supported catalysts, wherein at least a metal or metal compound is present in a selected part of the catalyst, i.e. having an inhomogeneous but non-random distribution of the metal or metal compound over the catalyst. It should be noted that metal-containing supported catalysts prepared by methods based on the conventional impregnation technique or based on a precipitation deposition in the presence of a hydroxyl ions-generating compound will generally have a more or less homogeneous distribution of the catalytically active compound on the support. In analogy herewith it would have been expected that a similar distribution would have been obtained when applying the hereinbefore mentioned precipitation deposition in the presence of a reducing agent to a preshaped carrier. The use of the precipitation deposition method in the presence of a reducing agent is known from NL-A-6816777. In said document a method for the preparation of metal supported catalysts is described wherein a reducing agent is contacted with an aqueous solution of a salt of a catalytically active element, and wherein a powdered carrier material has been suspended under conditions which result in a precipitation of a compound of said catalytically active element exclusively on the suspended carrier material. The metal compounds which may be employed in the process according to the present invention are water-soluble metal compounds based on catalytically active metals which are capable of participating in redox reactions in aqueous media, i.e. compounds based on metals which can occur in more than one valency in such an aqueous medium. Although probably most metals , with the exception of the alkali metals and the alkaline earth metals, can occur in more than one valency under these circumstances, there is a preference for metals of groups Ib, VIb, VIIb and VIII of the periodic system, such as Ag, Cu, Mo, W, Re, Mn, Fe, Co, Ni, Rh, Pd and Pt. In the present process the changing of the metal valency of the metal in the starting compound results in a metal compound having a much lower solubility in the aqueous medium than the starting compound, and which compound is preferably insoluble in the reaction medium. The water-soluble compounds include both species wherein the metal composes the cation of said compound, as well as those wherein the metal is present in the anion of said compound, i.e. the metal forms part of a coordinate anion group such as a coordinate anion group based on an oxyacid or halometallic acid of said metal. It is also possible that the metal is present as a complex cation such as an ammine complex. Reagents capable of changing the valency of the metal of the metal compounds which may be employed in the process of the present invention include both reducing and oxidizing agents and are preferably metal-free compounds and are preferably water-soluble. Suitable reducing agents include organic reducing agents such as aldehydes, monosaccharides such as aldoses and ketoses, bisaccharides such as maltose, lactose and dextrose, alcohols, polyphenols such as resorcinol, pyrogallol, hydroquinone or derivatives thereof. Nitrogen compounds such as hydrazine and hydroxylamine or derivatives thereof may also be employed as reducing agents, especially in systems requiring strong reducing agents. Suitable water-soluble oxidizing systems include hydrogen peroxide, nitrate ions, perchlorate ions and the like. It is also conceivable that the valency of the metal in the metal compound in the process of the present invention can be changed in the presence of a gaseous oxidizing agent such as O₂, O₃ and Cl₂ or a reducing agent such as H₂. Contacting the aqueous solution of the metal compound with a reagent capable of changing the valency of said metal is generally sufficient to initiate the metal valency-changing reaction hereinafter referred to as redox reaction. The overall net result of such a redox reaction will be that in addition to the formation of a metal compound wherein the valency of the metal is different to that of the starting metal compound, hydrogen ions or hydroxyl ions are generated. This generation of hydrogen or hydroxyl ions makes it possible to influence the reaction rate of the redox reaction by changing the pH of the reaction medium. In the case of a hydrogen ion-generating redox reaction, increasing the pH (e.g. addition of hydroxyl ions) will increase the reaction rate, while lowering the pH (e.g. addition of hydrogen ions) will decrease the reaction rate. With a hydroxyl ion-generating redox reaction the reverse will apply, i.e. increasing or lowering pH will decrease, respectively increase the reaction rate. A preshaped support which is acidic or basic vis a vis the aqueous mixture is understood to be a support having an iso electric point (IEP) which is lower, respectively higher than the pH of the aqueous reaction mixture. The IEP of a support material can be determined by numerous known methods such as electrophoresis. By contacting the aqueous reaction mixture containing the metal compound and the agent capable of changing the valency of said metal with a preshaped support it will also be possible , in analogy to what was stated hereinbefore, to influence the rate of the redox reaction by selecting a support having an IEP (sufficiently) different to the pH of the aqueous reaction mixture. As mentioned hereinbefore, by the process of the present invention a catalyst can be obtained wherein the outer surface and shell or outer layer of the catalyst is substantially free of catalytically active metal or metal compound, or wherein at least the concentration of the metal or metal compound in the shell or outer layer is considerably lower than in the inner parts and especially the centre of the catalyst. Said mode of distribution will hereinafter be referred to as egg-yolk distribution. Combinations which favour the formation of the egg-yolk distribution comprise: a hydroxyl ion-generating redox reaction with an acidic reacting support, and a hydrogen ion-generating redox reaction with a basic reaction support. Since each of theme hydroxyl ion- or hydrogen ion-generating redox reactions can be carried out in the presence of an oxidizing or a reducing agent, the number of possible combinations to arrive at an egg-yolk distribution is doubled. The rate at which the inhomogeneous deposition occurs can be influenced by lowering the concentration in the aqueous mixture of one or more of the reactants, lowering the reaction temperature, which operations will both reduce the rate of deposition and also by increasing the difference between the IEP of the support and the pH of the aqueous reaction mixture. In situations wherein the difference between the pH of the aqueous reaction mixture and the IEP of the support is insufficient and cannot be increased by the addition of an acid or base, e.g. in systems wherein as a result of such an addition the support would tend to dissolve it may be useful to buffer the pH of said mixture. In the case of a high rate of deposition it might be necessary to allow the redox reaction to progress during only a relatively short time in order to sustain an inhomogeneous distribution of the metal or metal compound. The deposition may terminate spontaneously when the proper reaction conditions have been applied. The preshaped supports which may be employed in the process of the present invention will generally be based on refractory oxides and include compounds such as SiO₂, γ-Al₂O₃, η-Al₂O₃, ZrO₂, Cr₂O₃, MgO, TiO₂, ThO₂, and mixtures thereof such as SiO₂.Al₂O₃. Furthermore, reaction products of such an oxide and a metal oxide such as a spinel e.g. MgAl₂O₄ may also be used. The preshaped supports may also be based on zeolite type materials such as faujasite, mordenite, ZSM-5, ferrierite, alumina phosphate and the like. The preshaped carriers employed in the process of the present invention may optionally contain one or more catalytically active metal or metal compounds. The actual shape of the support is not very critical as this will generally be determined by the ultimate application of the catalyst to be made, and includes spheres, cylinders, hollow cylinders, rings, trilobes, granules and the like. It is preferred that these porous preshaped supports have a pore volume of at least 0.1 ml/g and a surface area of at least 0.5 m2/g. It may be advantageous to employ preshaped supports which have been submitted to only a relatively mild heat treatment or calcination. The distribution of the catalytically active metal or metal compound in the shaped catalyst prepared by the process according to the present invention was determined with the aid of the Electron-Probe Microanalysis technique. The principle of this technique has been the subject matter of numerous publications such as in "Quantitative Electron-Probe Microanalysis", by V.D. Scott and G. Love, published by Ellis Harwood Ltd., 1983. Said analysis technique has been used extensively in a wide range of scientific disciplines, including metallurgy, solid phase physics, mineralogy and biology. When applied to catalysts prepared by a process according to the present invention, the metal distribution was determined by measuring the metal concentration, in a cross section of the shaped catalyst, along a line running from one side of the cross section to the opposite side thereof and preferably through the centre. Which catalyst cross section is actually employed in this analysis is not critical. It may be advantageous to select a cross section which provides a more or less symmetrical diagram of the metal distribution e.g. a cross section perpendicular to the longest axis of the shaped catalyst, if present. With the more complicated shaped catalysts such as trilobes, rings and saddles, it may not always be possible to select such a cross section. Moreover, it is also possible that the overall cross section of a shaped catalyst may be composed of more than one identical local cross section, such as may be the case with a ring-shaped catalyst. However, even under those circumstances it should pose no problem to select a cross section with the aid of which it is possible to demonstrate the metal distribution of that catalyst. A typical diagram of the metal distribution in a spherical catalyst having an egg-yolk distribution as defined hereinbefore, and based on by Electron-Probe Microanalysis is shown in Figure 1. Fields of application for the metal-containing supported catalysts having an egg-yolk metal distribution may be found for instance in residue conversion, wherein group VIb metals such as Mo and W on silica or combinations of group VIII and group VIb metals such as Ni-Mo on silica catalysts may advantageously be employed. Exhaust-gas purification would be another outlet for the use of an egg-yolk type catalyst e.g. Co or promoted Co on silica or Pt-Rh on γ-alumina. The invention will be further illustrated by means of the following Examples, wherein the following materials were used: Supports : γ-alumina extrudate, AC 300: diameter 1.5 mm, length 4-6 mm, pore volume 0.68 ml/g, surface area 250 m²/g, calcined 60h at 500°C. Silica spheres ex SHELL: diameter 1.5 mm, pore volume 1 ml/g, surface area 260 m²/g. Granulated silica ex Grace Davison: 0.6-1.4 mm granules, pore volume 1.2 ml/g, surface area 310 m²/g. Reagents : (NH₄)₆Mo₇O₂₄.4H₂O ammonium hepta molybdate (AHM) N₂H₅OH, hydrazine hydrate 80%, ex Merck (zur Synthese) acetic acid, ex Baker (analyzed) Cu(NO₃)₂.3H₂O ex Mallinckrodt. HNO₃, (65%) ex Baker. AgNO₃, 63.5% Ag, ex Schöne Edelmetaal B.V. Formaldehyde: aqueous solution stabilized with methanol, 37% CH₂O ex Baker. Ammonia: concentrated as suplied. Water: in all examples and at all stages demineralized water was used. Example 1 Preparation of a molybdenum on silica catalyst having an egg-yolk Mo-distribution Preparation of Mo-solution 36.8 G of AHM was dissolved in 900 ml water, concentrated ammonia was added until a pH=8.7 had been obtained (solution I) Preparation of N₂H₄-solution 250 Ml of N₂H₅OH was slowly neutralized with concentrated acetic acid (195 ml) to a pH=8.9. During neutralization the solution was cooled. 46.1 Ml of the neutralized solution was diluted with water to a volume of 100 ml and cooled to 0°C. (solution II). Catalyst preparation Solution II was added to solution I and the combined volumes were made up to 1300 ml by the addition of water. The resulting solution had a pH=8.8 and contained 15.4 g Mo/l and had a N₂H₄/Mo mol ratio=2.3. The temperature of this solution was kept at or below 10°C. The solution was circulated at a rate of 60 ml/min for several hours through a small glass vessel (approx. 60 ml) containing 25 g of silica spheres. The temperature in this vessel was maintained at 30°C. During circulation the formation of a brown deposit within the silica spheres could be observed which deposition appeared to be discontinued after the first hour of circulation. After separation by filtration the silica spheres were washed with water and dried at 120°C in air. By means of X-ray fluorence (XFR) the catalyst was found to contain 1.5 %w Mo. With the aid of Electron-Probe Microanalysis (EPM) the Mo-distribution was determined as depicted in Figure 2. Example II Preparation of a Mo on silica catalyst, having an egg-yolk Mo-distribution, via a quenching technique An aqueous Mo/N₂H₄ solution, having pH=8.5 and containing 71.9 g Mo/l and a N₂H₄/Mo molar ratio=2, was prepared by the method as outlined in Example 1. 6.7 G of silica spheres were soaked in 40.5 ml of this solution at 0°C. The precipitation deposition reaction started a few minutes after the addition of the silica spheres, and was allowed to progress for 15 minutes. Subsequently the reaction was quenched by the addition of an excess of water of 0°C. After washing with water and drying at 120°C in air XRF indicated the catalyst to have a Mo content of 0.66 %w. With the aid of EPM the Mo-distribution was determined as depicted in Figure 3. Example III Preparation of a Mo on silica catalyst having an egg-yolk Mo-distribution and a higher Mo content than catalyst of Example I The catalyst preparation as described in Example I was repeated with an aqueous Mo/N₂H₄ solution having a pH=8.5 and containing 15 g Mo/l and having a N₂H₄/Mo mol ratio=1.1. The temperature of the solution was maintained at 27°C while the temperature in the vessel containing the silica spheres was kept at 50°C. The Mo/N₂H₄ solution was circulated for 20h. Subsequently the silica spheres were separated by filtration, washed with water and dried at 120°C in air. XRF indicated the catalyst to have a Mo content=6.7 %w, while with EPM the Mo-distribution was determined as depicted in Figure 4. Example IV Preparation of a Mo on silica catalyst having an egg-yolk Mo-distribution by first impregnating the silica support with the aqueous Mo solution and subsequently introducing the N₂H₄ solution into the reaction medium 25 G of silica spheres were soaked in 75 ml of an aqueous Mo solution having a pH=8.7 and a Mo content=0.1 g Mo/ml and having been cooled to 0°C. Subsequently 20 ml of an aqueous, neutralized N₂H₄ solution (pH=8.5, N₂H₄ content=35 %w, T=0°C) was added. The temperature of the resulting mixture was increased from 0°C to 60°C at a rate of 3°C/h while rotating the mixture in a round bottom flask. After 20h the silica spheres were separated by filtration, washed with water and dried at 120°C in air. XRF indicated the catalyst to contain 16 %w Mo and with EPM the Mo-distribution was determined as depicted in Figure 5. Example V Preparation of a Cu on γ-alumina catalyst having an egg-yolk distribution Preparation of N₂H₄=solution (solution I) 107 Ml of N₂H₄OH was diluted with 50 ml water. The pH of the solution was brought down to 8.72 by a slow and dropwise addition of concentrated acetic acid (approximately 59 ml). Preparation of Cu-solution (solution II) 95.06 G of Cu(NO₃)₂.3H₂O was dissolved in 480 ml water. By a slow and dropwise addition of concentrated HNO₃, the pH of the solution was brought down to 2.0. The volume of the solution was made up to 500 ml, by the addition of water. Catalyst preparation 4.2 Ml of solution I was added to 60 ml of solution II, as a result of which under a simultaneously evolvement of one or more gaseous compounds, the reactor contents gradually turned into a green liquid phase and an orange-red precipitate. After standing for approximately 1 1/2h the precipitate was removed by filtration and the pH of the filtrate was found to be 2.09. Subsequently 10.0 g of γ-Al₂O₃ extrudate was added whereupon again a gaseous phase was evolved while the extrudate became orange-red, via an intermediate blue phase. After 2h the pH of the liquid phase was 2.43. The extrudate was separated by decantation washed with water and dried in air at 120°C. XRF indicated the catalyst to contain 1.8 %w Cu, while with EPM the Cu-distribution was determined as depicted in Figure 6. Example VI Preparation of an Ag on γ-alumina catalyst having an egg-yolk Ag-distribution Solution I: 5 g AgNO₃ dissolved in 75 ml water. Solution II: 0.87 g formaldehyde solution was mixed with 25 ml water. 15 Ml of solution I was diluted with 20 ml water and mixed with 6 ml of solution II. After bringing the pH of said mixture down to 0.94 by the addition of concentrated HNO₃, 1 g γ-alumina extrudate was added. The deposition reaction was allowed to proceed for 3 min. at 21°C. Subsequently the extrudate was separated by decantation of the liquid phase, and washed consecutively with a mixture of concentrated HNO₃ and water 1/1 v/v and water, and finally dried at 120°C in air. XFR indicated the catalyst to have an Ag content=0.38 %w while with EFM the Ag-distribution was determined as depicted in Figure 7.

Claims: Process for the preparation of a metal-containing supported catalyst wherein the concentration of at least one metal or metal compound is considerably lower in the outer layer of the catalyst than in the inner portion of the catalyst, comprising contacting an aqueous solution of an appropriate metal compound with a reducing agent or an oxidizing agent resulting in a redox reaction either (a) generating hydrogen ions during contact with a basic reacting preshaped support, or (b) generating hydroxyl ions during contact with an acidic reacting preshaped support. A process according to claim 1, wherein use is made of preshaped support based on one or more refractory oxides such as SiO₂, γ-Al₂O₃, η-Al₂O₃, ZrO₂, Cr₂O₃, MgO, TiO₂, ThO₂ or reaction products of metal oxides and such refractory oxides; or on zeolite materials such as faujasite, mordenite, ZSM-5, ferrierite and alumina-phosphate. A process according to claim 1 and/or 2, wherein the reducing agent is an organic reducing agent. A process according to claim 3, wherein the reducing agent is hydrazine, a derivative thereof, a hydroxylamine or a derivative thereof. A process according to claim 1 and/or 2, wherein the oxidizing agent is hydrogen peroxide.

Inventors: DE JONG KRIJN PIETER, DE JONG, KRIJN PIETER
Applicants: SHELL INT RESEARCH, SHELL INTERNATIONALE RESEARCH MAATSCHAPPIJ B.V.
Application year: 1987
Main classification: B01J37
IPC codes: B01J29, B01J37, B01J23