GB2189163A - Process for preparation of silica supported catalysts - Google Patents

Abstract

A process for the preparation of silica supported catalysts containing at least one catalytically active group VIb metal or metal compound, and at least one catalytically active group VIII metal or metal compound, wherein at least a substantial amount of said group VIb metal, metal compound or a precursor thereof is deposited on the silica carrier via chemical precipitation from a solution of a group VIb metal compound, which precipitation has been preceded by depositing by precipation on said carrier at least a compound of said group VIII metal, and drying the thus obtained carrier. The resulting catalysts demonstrate a high catalytic activity and may be used in hydrotreating processes as well as in water gas shift reactions and olefin disproportionation processes.

Description

SPECIFICATION Process for preparation of silica supported catalysts The invention relates to a process for the preparation of silica supported catalysts. More in particular, the invention relates to the preparation of silica supported catalysts containing catalytically active group Vlb and VIII metals or metal compounds.

Such catalysts may be employed in a large number of processes and are especially suitable for use in processes wherein hydrocarbon feedstocks are treated with hydrogen, such as hydrogenation, hydrocracking, hydrodesulfurization (HDS), hydrodemetallization (HDM) and hydrodenitrification (HDN). These processes are often referred to as hydrotreating processes. Other fields of application where such catalysts may advantageously be used include olefin disporportionation processes and the water gas shift reaction. Although the majority of the commercially available hydrotreating catalysts are alumina supported, there is a growing interest for the use of silica supported hydrotreating catalysts, as these might be less prone to coke formation presumably because of the lower acidity of the silica carrier; coke being a major catalyst deactivator in such processes.

The preparation of silica supported hydrotreating catalysts is known from the literature. In Applied Catalysis, 11(1984) 1-13, the preparation of (Ni/W) and (Ni/Mo) on silica catalysts is described via sequential impregnation of a powdered silica carrier material with small amounts of organometallic compounds of Ni, W or Mo. After completion of the first impregnation step the impregnated carrier may be subjected to a heat treatment in the presence of H2, 2 or hydrogen sulfide (H2S), the latter often being referred to as sulfiding step. Upon completion of the second impregnation step the impregnated carrier is subjected to a sulfiding step.The reported activity data of the different catalysts indicate that in order to obtain catalysts with the highest activity the carrier should be impregnated with the Mo or W compound prior to being impregnated with the Ni compound and moreover that a sulfiding step should be introduced between the two impregnation steps.

In United States patent specification 3167496 the preparation of silica support Ni/Mo catalysts is described, which catalysts contain up to 15%w of a compound of Mo, selected from the group consisting of oxide and sulfide and from 0.5 to 1.6%w of a compound of Ni, selected from the group consisting of oxide and sulfide. A suitable method for the preparation of these catalysts is based on the impregnation of a silica carrier with a precursor of a molybdenum catalystic agent prior to impregnation with a precursor of a nickel catalytic agent. In particular the molybdenum containing carrier is calcined before impregnation with the nickel compound takes place.Upon completion of the second impregnation step the carrier is calcined which calcination may be followed by a treatment with H2S to convert the metal oxides into the corresponding sulfides. It is explicitly stated therein, that catalysts prepared by the reverse order of impregnation are of inferior quality.

Surprisingly it has now been found that highly active silica supported catalysts can be prepared via a process wherein the silica carrier is contacted with a compound of a catalytically active group VIII metal prior to being contacted with a compound of a catalytically active group Vlb metal.

Accordingly the invention provides a process for the preparation of silica supported catalysts containing at least one catalytically active group Vlb metal or metal compound and at least one catalytically active group VIII metal or metal compound, wherein at least a substantial amount of said group Vlb metal, metal compound or a precursor thereof is deposited on the silica carrier via precipitation from a solution of a group Vlb metal compound, which precipitation has been preceded by depositing on said carrier at least a compound of said group VIII metal, and drying the thus obtained carrier.

In the context of the present invention whenever a group Vlb or a group VIII metal compound is in essence deposited on a silica carrier via precipitation it will be understood that said precipitation results from a chemical reaction in said precursor solution, and not as result of a physical process, such as evaporation or crystallization.

In precipitation-deposition methods for the preparation of supported catalysts, it appears to be extremely important that the rate at which the precipitating compound is transported to the carrier surface area, is sufficiently high to avoid and even prevent the formation of nuclei for precipitation in the solution, so that said precipitating compound will in essence only be deposited, on the carrier, which carrier is generally suspended in said solution. Hence the precipitation should be very gradual, and closely controlled.

When in the context of the present invention a group Vlb or group VIII metal or metal compound is mentioned it is understood that said group Vlb and group VIII refer to groups of the Periodic Table Of The Elements, as published in Perry's Chemical Engineers' Handbook, Sixth Edition.

The silica which may be used as carrier material in the process of the present invention includes both non-shaped silica carriers, such as the many types of commericaly available silica powders, as well as shaped silica carriers, such as silica spheres and granules. The surface area and pore diameters of these silica carriers are not critical per se and may vary over wide ranges and will primarily be determined by the end use of the ultimate catalyst.

The contacting of the silica carrier with a group VIII metal compound can be effected by known techniques. For instance, the silica carrier can be impregnated with a solution of a group Vlil metal compound, which solution may be an aqueous solution or based on an organic solvent. With the latter, the dissolved group VIII metal compound may be an organometallic compound, such as a Ni-allyl or Co-allyl-compound as described in Applied Catalysis, 11(1984) 1-13.Impregnation can generally be carried out by contacting the carrier with a volume of impregnation solution which is approximately equal to the total pore volume of the carrier and subsequently evaporating the liquid (impregnation to incipient wetness) which mode is preferred when using a shaped carrier, or alternatively dispersing the carrier in an excess of the impregnation solution and evaporating the liquid phase.

Group VIII metal compound supporting silica carriers can also be prepared via a precipitationdepostion methods as has been described in Dutch patent specifications 6705259 and 7005137. With this method the precipitation is effected in the presence of one or more compounds which are capable of generating e.g. upon heating, sufficient hydroxyl groups to precipitate the group VIII metal as a hydroxide or an insoluble salt. Suitable sources, which are capable of generating hydroxyl groups, include the hydrolysis of urea, the reaction of urea with nitrous acid and the hydrolysis of nitrite anions. In the process according to the present invention it is preferred to employ a non-shaped carrier when conducting the precipitation of a group VIII metal compound according to the hereinbefore described hydroxyl groups generating method.

A further method for the preparation of a group VIII metal containing silica carrier comprises introducing a catalytically active group VIII metal compound or a precursor thereof during the manufacturing of the silica carrier, such as silica spheres.

In the preparation of a catalyst by the process according to the present invention wherein a small part of the group Vlb metal compound may be deposited on the silica carrier together with or even before the group VIII metal compound, there is a preference for conducting said deposition via a co-impregnation step and more preferably via an aqueous co-impregnation step.

After having deposited the group VIII metal compound on the silica carrier, it may be advantageous in the process according to the present invention to subject the carrier to a heat treatment process step, especially when the group VIII metal compound had been deposited via an impregnation process step. To this end the carrier is, when appropriate, separated from the excess liquid in which it had been dispersed during said deposition and subsequently dried at a temperature of up to e.g. 200"C. The dried carrier may be subjected to a more severe heat treatment (calcination) to convert the group VIII metal compound into the corresponding metal oxide. Such heat treatments are generally carried out while the carrier is in contact with air.

Optionally, the heat treatment may be followed by a sulfiding step. When the group VIII metal compound has been deposited by means of an impregnation step, it is preferred that the carrier is subjected to a calcination step. However when a co-impregnation step has been applied, i.e.

an impregnation with a mixture of a group VIII metal compound solution and a group Vlb metal compound solution, the dried carrier is not normally calcined and may optionally be subjected to a sulfiding step. Said sulfiding step may be conducted at a temperature in the range of from 100"C to 500"C, by contacting said carrier with a mixture of e.g. H2/H2S, H2/carbondisulfide or H,/dimethyldisulfide. Said sulfiding may also be accomplished in-situ in a process wherein a sulfur containing feedstock is employed. Should the group VIII metal compound have been introduced during the manufacturing of the silica carrier, a separate calcination is not essential, as the preparation of a shaped carrier usually includes a calcination step as final process step.

Concomitantly the shaped carrier may be contacted as such with the group Vlb metal compound.

As mentioned hereinbefore the group Vlb metal compounds are deposited on the group VIII metal compound supporting-carrier via precipitation from a solution of a group Vlb metal compound. Such a precipitation method, wherein the precipitation is initiated by the generation of hydroxyl groups, has been described in the Dutch patent specifications cited hereinbefore.

Suitable group Vlb metal compounds which may be employed in such a method comprise watersoluble salts, wherein said metal salts are present as cations. When applying this method for the precipitation of said group Vlb metal compounds it is preferred to employ a non-shaped carrier.

An alternative method to achieve a gradual precipitation of a group Vlb metal compound comprises using a solution containing salts, e.g. ammonium salts, of the appropriate group Vlb metal oxy acids, and one or more compounds which are capable of generating protons. The principle whereon this precipitation-deposition is based, may be exemplified by the following reaction scheme:

Suitable such proton-generating systems include the hydrolysis of esters such as dimethylsulfate and dimethyloxalate and is exemplified for dimethylsulfate by the following reaction

This novel method has the important advantage that high pH values are avoided thereby preventing silica going into solution.

A third precipitation-deposition method which may be employed in the process of the present invention has been described in Dutch patent specification 6816777. In said method the gradual precipitation of the group Vlb metal compounds is effected by the presence in the solution of a reducing agent. Preferred reducing agents applicable in the process according to the present invention are strong reducing agents such as hydrazine N2H4 or derivatives thereof and the hydroxylamines or derivatives thereof. The group Vlb metal compounds which may conveniently be used in this precipitation-deposition method are the oxy acid salts of said metals, preferably the corresponding ammonium salts such as (NH4)2MoO4, (NH4)6,Mo7024.4H2O, (NH4)2WO4 and (NH4)6W7024.6H20.

In the process according to the present invention it is preferred to deposit substantially all of the group Vlb metal compound, via precipitation deposition on the group VIII metal compound supporting carrier.

Upon completion of the step wherein the carrier is contacted with the hereinbefore described group Vlb metal compound, the carrier is when appropriate separated from the liquid medium in which it had been dispersed, and subsequently dried at a temperature of up to 200"C. Depending on the ultimate use of the catalyst, the dried carrier thus obtained may be calcined.

Preferably this calcination is carried out at a temperature not exceding 350"C.

Alternatively the dried carrier may be subjected to a sulfiding step, which sulfiding step may optionally also be applied to the calcined group Vlb and group VIII metal compound supportingcarrier.

Sulfiding agents which may be employed and the temperatures at which the sulfiding may be conducted are similar to those which can be applied in the hereinbefore mentioned sulfiding of the group VIII metal compound(s) supporting carrier.

The metal content of the catalysts prepared by the process of the present invention will generally by in the range of from 0.1 to 20%w for the group VIII metal(s) and from 0.5 to 40%w for the group Vlb metal(s). Preferably the group VIII and group Vlb metal content is from 1 to 109sow and from 5 to 35%w respectively. In the process of the present invention for the preparation of silica supported group VIB and group VIII metal or metal compound containing catalysts there is a preference to employ Mo or W as Group Vlb metal and Co or Ni as Group VIII metal.

The invention will be further illustrated by the following Examples wherein the following carrier materials(A) have been used: A-1. Aerosil 300V, a powdered silica (ex Degussa) having a B.E.T. surface area of approximately 280 m2/g.

A-2. Silica spheres, (ex SHELL) having an average diameter of 1.5 mm, a pore volume of approximately 1 ml/g and a B.E.T. surface area of approximately 260 m2/g; and the procedures defined hereinafter were employed: B- 1. Determining the metal content of the catalyst.

The metal content of the catalyst was determined via X-ray fluoresence of the dried nonsulfided metal supporting carrier.

B-2. Sulfiding of the metal supporting carrier.

The sulfiding of the metal supporting carriers was conducted in the same equipment as used for the testing of the ultimate catalysts i.e. a tubular fixed bed reactor. The metal supportingcarriers were crushed to arrive at particles of 0.18-0.59 mm. 0.2 G of crushed carrier was introduced in said fixed bed reactor and a 6/1 v/v mixture of H2 and H2S was passed over said reactor bed at a flow rate of 63 ml/min. The temperature was raised from ambient to 350"C, at a rate of 10 C/min., and upon reaching 350"C, maintained at that temperature for approximately 1 hour.

B-3. Measuring the catalyst activity.

The catalyst activity was measured in a thiophene hydrodesulfurization (HDS) reaction, by passing 54 I/min of H2, containing 6 mol % of thiophene, over the sulfided catalyst-containing fixed bed reactor at 350"C, which catalyst had been sulfided as described hereinbefore in section B-2. The activity data reported was obtained after the HDS reactor had been on stream for 5 hours.The catalyst activity is expressed as a first-order rate constant with the dimensions of moles of thiophene converted per kg of catalyst per hour: kw and calculated as

or as a first-order rate constant with the dimension of volume of thiophene converted per mole of group Vlb metal per unit of time kvlb, and calculated as

wherein: x=conversion of feedstock fraction Xt=mole fraction of thiophene F=total flow rate (mole.h-l) W=catalyst weight (kg) Fv=total volumetric flow rate (m3.s-1) Wv,b=amount of group Vlb metal (mole) Example I Preparation of a shaped silica supported Ni/Mo catalyst, wherein Ni is deposited via ari impregnation step and Mo via a reduction-initiated precipitation step a) Impregnation of silica spheres with Ni compound solution.

17.5 G ammonium carbonate was dissolved in 75 ml of concentrated ammonia (25 %w NH3) which had been diluted with 10 ml water. Next 25 g nickel carbonate (ex Borchers) was added and allowed to dissolve which required about 10 min. at 45-50"C. 15 Ml of the resulting darkblue nickel-ammonia solution (S), which contained 9.9 %w Ni was mixed with 15 ml ammonia (2.5 %w). 24 G of this mixture was required to impregnate 25 g of silica spheres to incipent wetness. After slow drying the thus obtained carrier was calcined at 500"C for 2 hours.

b) Precipitation of Mo on Ni supporting silica spheres.

Preparation of Mo solution: 176.5 G of (NH4)6Mo7024-4H2O (AHM) was dissolved in water and the pH was adjusted to 8.8 with ammonia (25 %w), and the total volume of the solution was made up to 1000 ml with water. This solution contained 0.096 g Mo ml-'.

Preparation of hydrzine solution: 200 ml of a concentrated N2HsOH (Merck, 80%w) solution was neutralized with concentrated acetic acid and diluted with water; the solution contained 41 %w N2H4 and the pH was 8.7. In a round bottom flask 10 g of the nickel-loaded spheres was contacted with 30 ml of the Mo solution. The mixture was cooled down to 0 C. 7.5 ml of the hydrazine solution was diluted to 10 ml and also dooled down. The hydrazine solution was added to the mixture. Subsequently the flask was continuously rotated while applying a nitrogen purge.After lh at 0 C the reaction mixture was heated at a rate of 3 C.h-1 to a maximum temperature of 54"C and was kept at that temperature for lh. Subsequently the loaded carrier was separated from the liquid by filtration, washed and dried at 120"C for 16h. The dried product contained 2.6 %w Ni and 12.5 %w Mo (determined via method B-1).

Example II Preparation of a shaped silica supported Ni/Mo catalyst as in Example I but having a higher Ni and Mo content Following the same procedures as described in Example I 50.5 of silica spheres were impregnated to incipient wetness with 50.3 g of a Nickel-ammonia solution obtained by diluting 34 ml of Ni solution (S) to 51 ml by the addition of ammonia (25 %w). After slow drying followed by calcination at 500"C for 6 hours, 6.7 g of the calcined Ni supporting carrier was contacted with 30 ml of a similar aqueous Mo solution as in Example 1(0.096 g Mo/ml, pH=8.8) and after cooling 10 ml of a similar hydrazine solution (17 %w N2H4; pH=8.0; 0 C) was added. After 1 hour at 0 C the flask contents were heated to 60"C at a rate of 3.5OC/hour and subsequently held at that temperature for 1 hour. The ultimate dried product contained 4.2 %w Ni and 17.0 %w Mo.

Example III Preparation of a silica powder supported Ni/Mo catalyst, wherein Ni is deposited via a hydroxyl ion initiated precipitation and Mo via a reduction-initiated precipitation 5.2 g Ni(No3)2.6H20 was dissolved in 500 ml water and nitric acid was added to a pH=2.0 20 g of Aerosil 300V was suspended in the solution. After the addition at ambient temperature of 7.5 g urea the suspension was heated to 90"C at a rate of 1.6 C/min and subsequently kept at that temperature for 20 hours. After filtration and washing with water, approximately half of the Ni supporting silica was transferred back to the reactor.Subsequently a solution of 9 g AHM in 150 ml water (pH adjusted with ammonia to 8.7) and 30 g of N2HsOH in 150 ml water (pH adjusted to 8.0 with acetic acid) was introduced into the reactor at ambient temperature and the volume of the contents made up to 500 ml by the addition of water. The reactor contents were heated from ambient temperature to 90"C under stirring and a N2 flow at a rate of 3.50hour and subsequently kept at that temperature for 4 hours. After filtration and washing with water the metal supporting carrier material was dried in air at 120"C. The dried material contained 8.4 96w Ni and 31.0 %w Mo.

Example IV Preparation of a silica powder supported Co/Mo catalyst wherein Co is deposited via a hydroxyl ion initiated precipitation and Mo via a reduction-initiated precipitation The same procedures as described in Example lil were followed but using 4.9 g Co( NO3)2-6H20, 3 g urea and 10 g of Aerosil 300V and reacting at 90"C for 16 hours. After filtration and washing all the Co supporting silica was used for the Mo deposition for which process step a solution of 6 g AHM in 250 ml water (pH adjusted to 8.7 with ammonia) and 20 g N2H50H in 250 ml water (pH adjusted to 8.0 with acetic acid) was used. After filtration and washing with water the metal supporting catalyst was dried at ambient temperature under reduced pressure (200 mbar). The dried material contained 6.3 %w Co and 15.8 %w Mo.

Comparative Experiment A Preparation of silica powder supported Co/Mo catalyst wherein Mo was deposited on the carrier via reduction-initiated precipitation before Co was deposited via a hydroxyl ion initiated precipitation In a round bottom flask 10 g of Aerosil 300V was suspended at ambient temperature in a mixture of a solution of 6 g AHM in 250 ml water (pH adjusted to 8.6 with ammonia) and 6 g N2HsOH in 250 ml water (pH adjusted to 8.0 with acetic acid). The suspension was heated from ambient temperature to 90"C under stirring and N2 flow, at a rate of 600hour and kept at that temperature for 3 hours. After filtration the carrier was washed with water. Approximately half of the Mo supporting carrier was transferred to the reactor.Subsequently 500 ml water, 2 g Co(NO3)2-6H2O and 0.85 g urea were added to the reactor. The resulting suspension was heated to 90"C while stirring and under a N2 flow, at a rate of 1.6 C/min. and kept at that temperature for 18 hours. After filtration and washing with water the metal supporting carrier was dried in air at 1200C. The dried product contained 3.2 %w Co and 6.2 %w Mo.

Comparative Experiment B Preparation of a shaped silica supported Mo catalyst wherein Mo was deposited via a reductioninitiated precipitation Following the same procedure as in Example I for the deposition of Mo 25 g of silica spheres was contacted with 75 ml of an aqueous AHM solution (0.095 g Mo/ml, pH=8.7) followed by the addition of 20 ml of a hydrazine solution (35 %w N2H4, pH=8.3, T=0 C).

The ultimate dried product contained 16.0 %w Mo.

The catalysts as prepared according to Examples l-IV and according to the comparative experiments A and B were sulfided and subsequently tested for thiophene conversion activity, following method B-2 as hereinbefore described. The resulting data (obtained following method B-3) has been collected in Table 1.

This data demonstrates that the catalysts prepared according to the process of the present invention have a higher activity for converting thiophene than a catalyst prepared by depositing the metals in the reverse order and are also more active than a comparable catalyst containing only a group Vlb metal compound.

TABLE 1

Example Composition %w Co/Ni %w Mo/W Deposition Carrier Thiophene CATALYST Activity Sequence Type Conversion % kW1) kVIb2) x 106 I Ni/Mo 2.6 12.5 Ni first spheres 35 21 108 II Ni/Mo 4.2 17.0 " " 48 32 121 III Ni/Mo 8.4 31.0 " powder 55 38 80 IV Co/Mo 6.3 15.8 Co first " 21 11 46 Comparative Experiment A Co/Mo 3.2 6.2 Mo first powder 2.3 1.1 12 B Mo - 16.0 - spheres 4* 2.0 8 1) mol T.kg-1.h-1 2) m T.mol-1.s-1 * Value obtained by extrapolation of corresponding thiophene conversion data obtained at lower space velocities.

Claims (27)

1. A process for the preparation of silica supported catalysts containing at least one catalytically active group Vlb metal or metal compound and at least one catalytically active group VIII metal or metal compound, wherein at least a substantial amount of said group Vlb metal, metal compound or a precursor thereof, is deposited on the silica carrier via precipitation from a solution of a group Vlb metal compound, which precipitation has been preceded by depositing on said carrier at least a compound of said group VIII metal, and drying the thus obtained carrier.

2. A process according to claim 1, wherein the carrier is a shaped silica carrier.

3. A process according to claim 1 or 2, wherein a group VIII metal compound is deposited on the carrier via impregnation of the carrier with a solution of said metal compound.

4. A process according to claim 3, wherein the group VIII metal solution is an aqueous solution.

5. A process according to claim 3 or 4 wherein the solution of the group VIII metal compound also contains a group Vlb metal compound.

6. A process according to claim 1, wherein the group VIII metal compound is deposited on the carrier via precipitation in the presence of one or more compounds capable of generating hydroxyl groups.

7. A process according to claim 6, wherein the carrier is a non-shaped silica carrier.

8. A process according to any one of claims 2 to 5, wherein a group VIII metal or metal compound is introduced during the manufacturing of said shaped carrier.

9. A process according to any one of the preceding claims, wherein at least a group VIII metal compound supporting carrier is dried at a temperature of up to 200"C.

10. A process according to claim 9, wherein the dried carrier is calcined.

11. A process according to any one of the preceding claims, wherein the precipitation of the group Vlb metal compound is carried out in the presence of one or more compounds capable of generating hydroxyl groups.

12. A process according to claim 11, wherein the carrier is a non-shaped carrier.

13. A process according to any one of claims 1 to 10, wherein the precipitation of the group Vlb metal compound is carried out in the presence of one or more compounds capable of generating protons.

14. A process according to claim 13, wherein an alkyl ester of an organic or inorganic acid is used to generate protons.

15. A process according to any one of claims 1 to 10, wherein the precipitation of the group Vlb metal compound is carried out in the presence of a reducing agent.

16. A process according to claim 15, wherein the reducing agent is hydrazine, a derivative thereof, a hydroxylamine or a derivative thereof.

17. A process according to any one of claims 13 to 16, wherein an ammonium salt of an oxy acid of said group Vlb metal is used.

18. A process according to any one of the preceding claims, wherein substantially all of the group Vlb metal compound is deposited on the silica carrier via precipitation.

19. A process according to claim 1, wherein the dried carrier is calcined.

20. A process according to claim 19, wherein the calcination is conducted at a temperature not exceding 350"C.

21. A process according to claim 1 or 20, wherein the group Vlb and group VIII metal compound supporting carrier is sulfided.

22. A process according to any of the preceding claims wherein a silica supported catalyst is prepared having a group VIII metal content in the range of from 0.1 to 20 %w and a group VIB metal content in the range of from 0.5 to 40 %w.

23. A process according to claim 22, wherein a catalyst is prepared having a group VIII metal content in the range of from 1 to 10 %w and a group VIB metal content in the range of from 5 to 35 %w.

24. A process according to claim 23, wherein the group Vlb metal or metal compound is Mo or W or a compound thereof and the group VIII metal is Co or Ni or a compound thereof.

25. A process according to claim 1, substantially as hereinbefore described with particular reference to the Examples I to IV.

26. A catalyst whenever prepared by a process according to any one of the preceding claims.

27. A hydrotreating process employing a catalyst prepared by a process according to any one of claims 1, 19 or 21.